Printed Circuit Board Identification Using 1-Wire® Products
Abstract: There are two ways to identify a printed circuit board (PCB). The 'minimalist' approach is based on devices using ROM technology. The 'nameplate' approach requires user-programmable NV memory to store the PnP-related information. The most obvious way to implement Plug-and-Play capabilities is by means of a memory chip that holds the required information and does not lose it if power is switched off. 1-Wire chips for PCB identification are available in ROM, EEPROM, and EPROM (One-Time-Programmable) technology.
Introduction
Anyone who has ever tried to add a modem- or network-card to a PC knows that such an undertaking may produce unwanted results and can take more time than expected. To achieve cost savings from high volume production, the add-in board is designed to be logically compatible to a large variety of motherboards from various vendors. This flexibility makes it necessary to configure the board for its operating environment before it is ready to function. To perform the configuration one needs two pieces of information: sufficient knowledge of the system that the board is expected to reside in and knowledge of the board itself. Once the suitable settings are determined one needs to set jumpers or flip switches on the board. This completes the tricky part of the hardware installation.
As long as such upgrades or changes are made routinely and by trained personnel, they are time-efficient and do not involve much risk. Every time one has to deal with an unfamiliar product, a new learning cycle begins with studying the associated documentation (which may not be well-written) and a phase of trial and error, which may result in damage to the product or the system it is supposed to work in. In 1994, the concept of "Plug-and-Play" (PnP) was introduced to get out of this awkward situation, particularly with PCs. In order to function, each PnP device must have all of the following capabilities:
It must be uniquely identified.
It must state the services it provides and the resources it requires.
It must identify the software driver that supports it.
Although these requirements were originally specified for PC products, all electronic systems that consist of more than one device (or circuit board) do benefit from Plug and Play capabilities.
The most obvious way to implement PnP capabilities is by means of a memory chip that holds the required information and does not lose it if power is switched off. In some cases, suitable memory is already on the board (e.g., inside a microcontroller or as a byte-wide memory chip or serial EEPROM). In other cases or if the entire memory is already assigned to other functions, an additional memory chip is needed. Due to their smaller physical size and minimal additional I/O requirements, serial memories are preferred over byte-wide memories. Of all serial memories, devices with 1-Wire interface are easiest to interface because they require only a single data line plus ground reference. Most 1-Wire devices do not even have a VCC pin; they draw the energy for operation right from the data line. All 1-Wire devices have a unique registration number, which allows for traceability and performs as an address if multiple 1-Wire devices (not just memories) are connected in parallel to form a 1-Wire net. This feature is not found with other serial devices. Parts of the registration number can be customized, which turns a 1-Wire device into a custom chip that authenticates the board and makes it impractical to clone.
Spare memory bytes not used for PnP functions are available to enable/disable some of the resources of the circuit board, opening the door to license management rather than changing hardware for upgrades. Depending on storage capacity and chip technology, the memory could hold manufacturing data for quality control, warranty information, and repair history. Having such information readily accessible reduces the time to repair.
Device Technologies
1-Wire chips for printed circuit board identification need to maintain stored data independent of any power supply. This limits the choice to devices that are manufactured in nonvolatile (NV) technologies such as ROM, EEPROM, and EPROM (Table 1). The lack of a window in the plastic package or the mounting technique of chip scale packages with the active side towards the circuit board converts EPROM chips to One-Time-Programmable (OTP) memories.
Table 1. 1-Wire Devices For Circuit Board Identification
Part #
Technology
Memory
Appl. Type
Voltage
Packages
Extras
DS2401
ROM
64 bits
Minimalist
2.8V to 5.5V
Plastic, CSP
-----
DS2430A*
EEPROM
32 + 8 bytes
Nameplate
2.8V to 5.5V
Plastic, CSP
8 bytes OTP
DS2432
EEPROM
128 + 16 bytes
Nameplate
2.8V to 5.5V
Plastic, CSP
Challenge & response
DS2433
EEPROM
512 bytes
Nameplate
2.8V to 5.5V
Plastic, CSP
-----
DS2406
OTP EPROM
128 bytes
Nameplate
2.8V to 5.5V, 12V write
Plastic, CSP
2 controlled I/O pins
DS2502
OTP EPROM
128 bytes
Nameplate
2.8V to 5.5V, 12V write
Plastic, CSP (excl. UNW)
UniqueWare™ version
DS2505
OTP EPROM
2048 bytes
Nameplate
2.8V to 5.5V, 12V write
Plastic
UniqueWare version
DS2506*
OTP EPROM
8192 bytes
Nameplate
2.8V to 5.5V, 12V write
Plastic
UniqueWare version
*The DS2506 and DS2430A are no longer recommended for new designs.
Circuit Board Identification Overview
As indicated in column APPL. TYPE of Table 1, there are two ways to identify a printed circuit board. The minimalist approach is based on devices using ROM technology. The nameplate approach requires user-programmable NV memory to store the PnP-related information. Table 2 shows the identification alternatives of both approaches. Reference numbers beginning with 1 indicate the minimalist approach. For the nameplate approach there are devices in EEPROM technology (reference numbers beginning with 2) and OTP EPROM (numbers beginning with 3). Within each of these groups, the reference letter A stands for generic, off-the-shelf parts. The letter B is used for parts that have a customized ROM. The ROM customization is explained in section Minimalist Approach. The letter C indicates special features such as protected memory (2C) or factory pre-programmed parts (3C). For more details on this see section Nameplate Approach.
With customized devices, the cloning protection is achieved by restricted supply. Clone manufacturers will not be able to buy the customized parts. To achieve protection with generic devices (excluding the DS2432) it is necessary to manipulate the data, e.g., by means of encryption.
Common encryption methods (e.g., cipher block chaining) rely on a key ("secret") and an initialization vector. Embedding the unique ROM registration number of the memory chip in the initialization vector ensures that identical source data looks different for every individual device. This way, if data is copied from one chip to another, the copy will not work in the application. Details about encryption algorithms and software are available on the Internet, for example at the National Institute of Standards and Technology website.
*There may be a setup fee for custom parts, which is not included in this cost comparison.
Minimalist Approach
The lowest-cost method of identifying circuit boards (1A) uses generic (non-customized) DS2401 silicon serial numbers and relies on a central database that associates the ROM registration number of the 1-Wire chip to an individual board. Due to lack of user-programmable memory, this approach does not support PnP.
A somewhat more costly alternative (1B) uses devices with a customized ROM registration number. This allows the customer (board manufacturer) to specify 8, 12, or 16 bits of the 64-bit ROM content (Figure 1). Having control over some of the ROM bits allows for limited PnP compliance. A central database is still required to associate the ROM content to an individual board. For PnP to function, the circuitry on the board must be able to read the 1-Wire device and be able to forward the information to host CPU.
Since the customized chips are only sold to the original customer and its authorized agents, they are controlled products that are not available for clone manufacturers. To take advantage of this, the firmware must be able to distinguish between legitimate customized chips and generic chips or customized chips made for other customers.
Generic devices (1A) are easy to order and, in small quantities, are usually shipped from stock. Customized devices (1B) are manufactured on order and have a lead-time of 13 weeks. Contact Maxim or its sales organization for a copy of the registration form.
The typical chip for the minimalist approach is the DS2401.
Minimalist Approach—No User Memory—Comparison Table
1A
Generic Device
Method
Database lookup based on registration number.
Precondition
Database that uses registration number as key to access detailed product information.
Administration
Enter registration number of every board into database and associate (reference to)
detailed product information.
Cloning Protection
None
Advantages
Lowest chip cost.
Disadvantages
Database automation is limited to the tester reading the registration numbers of a batch
of boards and then transmitting them as a list to the database; the association to the
product information is created manually.
Requires access to (remote) database to read a board's detailed information.
1B
Customized Device
Method
Database lookup based on registration number.
Board types or board versions are distinguished by different codes in the customer-specified
section (subfield) of the registration number.
Precondition
Database that uses registration number as key to access detailed product information.
Administration
Enter registration number of every board into database and associate (reference to)
detailed product information.
A portion of the product information can automatically be
identified by the content of the customer-specified bits (subfield).
Cloning Protection
Yes
Advantages
The chip is less costly than memory chips for the nameplate approach.
Less prone to human error, provided that the right chip is mounted on the board.
If different products or product versions require different test programs, the name of test
program can be used to verify that the right chip was mounted on the board.
Disadvantages
Database automation is limited to the tester reading the registration numbers of a batch
of boards and then transmitting them as a list together with test program name to the
database.
Requires access to (remote) database to read a board's detailed information.
Logistics for stocking and using different customized chips.
Figure 1. Generic and customized ROM registration number.
Option A: 268 million serial numbers per subfield value
Option B: 16.7 million serial numbers per subfield value
Option C: 1.04 million serial numbers per subfield value
Customized DS2401, customer ID 123 hex, option A, subfield contents AB hex
Hex representation of sample registration number: E5 12 3A B0 00 56 1D 81
The most significant bit of the customized family code is always 1.
From reading a customized ROM one cannot tell what the selected option is. However, for a particular customer identification code and device, the option remains the same for all variations of the user-defined subfield content.
Nameplate Approach
All devices suitable for the nameplate approach have user-programmable NV memory to store the PnP-related information. For the nameplate approach it is essential to list the data items that are required for PnP to function, to define a way to identify each data item, and to decide on a format in which the (numerical, logical, text) value of each data item is represented. The format may be customized for a particular application to save space or one may use an industry-standard, but larger, format like XML (extendible markup language). Further data items can be added to the list, provided that they can be identified and a format has been selected. The data items then can be stored one after another in any sequence in the memory of the board identification chip. Alternatively, one can embed the data between a length byte at the beginning and a CRC check at the end, and store it as a data file. More information on a
file format for 1-Wire devices is found in application note 114, "1-Wire File Structure."
Depending on the memory size (in bytes) there may be space for other information, such as manufacturing data. Since the information is stored in a chip on the board, there is no real need to access a central database to get information about the board. However, it may still be good practice to maintain such a database to store information that does not fit into the memory chip or is intentionally kept away from public access (e.g., bill of material, names of component vendors). As with the minimalist approach, the 64-bit ROM registration number is the key to that database.
The price of the identification chip depends on the storage capacity (memory size) and the technology it is manufactured in. As shown in an example later in this document, the information that is essential for PnP fits easily into 128 bytes or even less. If storing of additional information is considered, the 128 bytes of the small memory chips may not be enough. Conversely, with a 2048-byte chip there is plenty of room to store an entire bill of material of more than 150 items in a compact format. Using the less storage-efficient XML tagging format, 2048 bytes may still be enough to include a bill of material of up to 50 items.
Suitable memory chips for the nameplate approach are manufactured in two technologies: EEPROM and OTP-EPROM. In either technology there are generic chips (referenced as 2A for EEPROM and 3A for EPROM) as well as ROM-customized chips (2B and 3B, respectively). The ROM customization is the same as described for the minimalist approach (see Figure 1) and the same restrictions and recommendations apply.
The protected EEPROM (2C) is customized by the user by installing an 8-byte secret in the device. Once installed, the secret—in contrast to a password—is never exposed on the 1-Wire line. When writing to a protected EEPROM, the host computer needs to send a 160-bit message authentication code, which is calculated from the secret and other data. When reading the EEPROM, the device can also generate an authentication code that allows the master to verify whether the chip knows the correct secret without ever exposing the secret itself.
UniqueWare devices (3C) are a special case of ROM customization. The customer-specific programming takes place in one or more of the first four memory pages. It always includes a Project ID (to distinguish between data patterns and customers) and a customer-specified serialization. UniqueWare is subject to the same restrictions as devices with customized ROM. For more information on UniqueWare refer to application note 99, "UniqueWare™ Project Setup Manual Revision 2.00B."
Generic devices (2A, 3A, and 2C) are easy to order and in small quantities are usually shipped from stock. Customized devices (2B, 3B) are manufactured on order and have a lead-time of 13 weeks. Contact Maxim or its sales organization for a copy of the registration form. UniqueWare devices (3C) are also programmed on demand. However, since the customization takes place at a packaged part, the lead-time is shorter than for ROM-customized parts.
The smallest EEPROM chip for the nameplate approach (2A, 2B) is the DS2430A. As a special feature, this chip, in addition to 32 bytes re-programmable memory, has an 8-byte one-time programmable register. The DS2433 has a storage capacity of 512 bytes, organized as 16 pages of 32 bytes each. The DS2432 protected EEPROM (2C) can store 128 bytes (4 pages of 32 bytes) of application data and one 8-byte secret. There are eight more registers for control functions. In EPROM technology (3A, 3B) there are four devices to chose from: DS2502 (128 bytes), DS2505 (2048 bytes), DS2506 (8192 bytes), and DS2406 (128 bytes). All these devices are organized in memory pages of 32 bytes. In addition to memory, the DS2406 has two extra pins that under software control can function as open-drain output (actuator) or as digital input (sensor). The DS2502, DS2505, and DS2506 are also available in UniqueWare versions (3C).
Nameplate Approach—EEPROM—Comparison Table
2A
Generic Device
Method
User-programmable memory chip stores relevant PnP data on the circuit board. Optional: Database lookup based on registration number for additional data.
Precondition
Definition of PnP data, additional data (as desired), data representation, and formatting.
Board tester that programs memory chip at final test. Optional: Database to access additional product information.
Administration
Ensure that the board tester uses the correct software. Optional: Create link between tester and database to store board registration number,
copy of chip data, and board test results.
Cloning Protection
No, unless data is encrypted.
Advantages
Highly automated database.
If the memory chip was programmed with incorrect data, it can be reprogrammed.
Disadvantages
More costly than minimalist approach.
Unless encrypted, data is not safe from unauthorized changes.
2B
Customized Device
Method
(Same as 2A)
Precondition
(Same as 2A)
Administration
(Same as 2A)
Cloning Protection
Yes, encryption is optional.
Advantages
(Same as 2A)
Disadvantages
More costly than generic device.
Unless encrypted, data is not safe from unauthorized changes.
2C
Protected Device (Customization by User)
Method
(Same as 2A)
Precondition
Board tester, which at final test installs a secret in the memory chip and programs it with the desired data.
(Otherwise same as 2A)
Administration
Ensure that the board tester uses the correct software and the correct secret.
(Otherwise same as 2A)
Cloning Protection
Yes, encryption is optional.
Advantages
Data can be read but not altered without knowing the secret.
The secret can be write-protected.
(Otherwise same as 2A)
Disadvantages
More costly than minimalist approach.
Nameplate Approach—OTP EPROM—Comparison Table
3A
Generic Device
Method
(Same as 2A)
Precondition
Board tester with 12V capability that programs memory chip at final test.
(Otherwise same as 2A)
Administration
(Same as 2A)
Cloning Protection
No, unless data is encrypted.
Advantages
Highly automated database.
If the memory chip was programmed with incorrect data, incorrect data can be invalidated
and new data can be written to an unused memory section (if available).
Data can be write-protected in blocks of 32 bytes.
Disadvantages
More costly than minimalist approach.
Requires 12V for writing to the memory chip.
3B
Customized Device
Method
(Same as 2A)
Precondition
(Same as 3A)
Administration
(Same as 2A)
Cloning Protection
Yes, encryption is optional.
Advantages
(Same as 3A)
Disadvantages
More costly than generic device.
Requires 12V for writing to the memory chip.
3C
Partially Preprogrammed Device—UniqueWare
Method
Customized chip is preprogrammed and partially write protected at the chip factory.
Remaining user-programmable memory stores relevant PnP data on the circuit board.
Optional: Database lookup based on registration number for additional data.
Precondition
Definition of the serialization format and constant data that is pre-programmed at the
chip factory.
(Otherwise same as 3A)
Administration
(Same as 2A)
Cloning Protection
Yes, encryption is optional.
Advantages
(Same as 3A)
Disadvantages
(Same as 3B)
Nameplate Design Considerations
The electronic nameplate has to include all data items that are required for PnP, a means to identify each data item, and a value for each item in a suitable format. Space permitting, there may be additional information added to the nameplate. As with metal or paper nameplates, one might want to include the company's name and address information. Since OTP-EPROM bits can only be programmed from 1 to 0, a format definition that allows changing programmed data under such constraints is desirable.
The following sample design concept meets these requirements. The nameplate begins with an ASCII string of the company name and website address, followed by a 00 byte as delimiter. All subsequent data is organized as groups. Each group contains data items for a particular purpose. Each data item has an identification number (1 to 255) that is unique within the group. The length of all data item values within a group is the same. Each group begins with an introductory byte that identifies the group number (1 to 15) and the value length (1 to 15 bytes) that applies to the group. The group number is stored in the higher nibble of the introductory byte, leaving the lower four bits to specify the value length. After the
last value of a group there is a 00h byte as delimiter.
The viability of this definition is demonstrated on the TINI®-board, a miniature web server of the size of a SIMM board. The TINI server consists of approximately 65 components, including a microprocessor. Some components and jumpers are installed only on certain versions of the board. The data is organized into two groups, manufacturing data and PnP data, as shown in Table 3.
Table 3. Sample Nameplate Contents
Group
Function
Items
Ident. #
Length
Format
1
Manufacturing
Data
Product name
Part number
Hardware revision
Date code/lot number
The manufacturing data includes everything that is typical for integrated circuits. All this information can be represented as ASCII text of no more than 12 bytes for each item.
The PnP data consists of five parameters: 1) number of RAM chip select lines, 2) size of each RAM chip, 3) presence of Ethernet chip, 4) connection of Ethernet chip to SIMM pins, and 5) presence of nonvolatizer. The first two of these parameters have a numeric value; the three others are "Boolean", which means they can be either true or false. Although these Boolean parameters could be merged into three bits of a single byte, this is not done in this example. Instead, a full byte FFh for "true" and 00h for "false" is used. The full nameplate including company name will look like this:
(C)MAXIM www.ibutton.com<00>
<1C><01>TINI SERVER <02>90-TINI1-512<03>REVB1 030900<04>0023B/117989<00>
<21><01><01><02><04><03><FF><04><FF><05><FF><00>
Information between angled brackets <> represents bytes in hexadecimal format. All the other information is plain ASCII text. The whole nameplate requires just 106 bytes, which easily fit in a DS2502 chip. If a bigger memory chip is chosen, one could include a full parts list or bill of material in the nameplate. For this purpose the parameter groups could be organized like this:
Group
Function
Length
Format
1
Manufacturing data
12 bytes each
ASCII
2
Plug-and-play data
1 byte each
(varies)
3
Batteries
9 bytes each
ASCII
4
Capacitors
9 bytes each
ASCII
5
Diodes
9 bytes each
ASCII
6
Transistors
9 bytes each
ASCII
7
Resistors
9 bytes each
ASCII
8
ICs part 1
9 bytes each
ASCII
9
ICs part 2
12 bytes each
ASCII
10
Crystals
9 bytes each
ASCII
11
Circuit board
9 bytes each
ASCII
Applied to the same example board, all this information would fit into less than 1kB. For this estimate, a proprietary part numbering system was chosen that assigns a 12-character part number to anything including component parts. Since the first three characters of the component parts are always the same they were omitted, reducing the length to nine characters. The part numbers of the integrated circuits in group nine start with a different code. For this reason the full length of 12 was used.
Since its identification number precedes each item within a group, the sequence of items is not critical. The sequence of groups can be changed also, because each group is identified by its introductory byte. This flexibility allows invalidating a group or an individual parameter within a group and to redefine it later. This is of particular interest with OTP-EPROM devices. Changing the group number to 0 invalidates a group; don't change the bits that specify the length of parameters within the group. A new copy of the whole group is then appended after the last parameter group. To invalidate and redefine a parameter, change the parameter number to 00 at the original location and append a new version of the parameter group with the new parameter value. It is not necessary to also include those parameters of the group that have not changed. Invalidation and redefinition of values or groups is not necessary with EEPROM devices. If anything changes, the entire data can be re-written.
The sample design concept described above is one way to create a nameplate. The mix of binary and ASCII data minimizes memory consumption, but is not standardized. Less memory-efficient but standardized is the XML format, which uses ASCII codes only. In XML format, the same example may look like this:
This XML file is 380 bytes long; formatted in 1-Wire file structure as described in application note 114, this XML nameplate will fill 15 of the 16 memory pages of a DS2433 chip. The size of the XML file can be reduced by choosing shorter abbreviations for each parameter.
Obtaining a Conventional Board Serial Number
Despite having an electronic board identification or nameplate, there may be a desire to also print a conventional, human-readable serial number on the circuit board. A hexadecimal form of serial number is already included in the 64-bit ROM registration number. If the nameplate approach is chosen, one can embed a decimal serial number in the design of the nameplate. UniqueWare, as a special case of the nameplate approach, already includes a customer-specified serialization in the factory-programmed memory section. Altogether there are four cases to be distinguished:
To get a conventional serial number, take the 48-bit serial number (see Figure 1) and convert it into a decimal number. Since the DS2401, the typical chip of the minimalist approach, is produced in large quantities the resulting decimal number is 9 or 10 digits long.
Customized Silicon, Minimalist and Nameplate Approach (References 1B, 2B, and 3B)
To get a conventional serial number, take the content of the serialization section and convert it into a decimal number. As shown in Figure 1, the serialization section of customized parts does not exceed 28 bits. This limits the length of the serial number to 9 decimal digits (option A) and 8 digits under option B. Depending on the forecasted demand, a 7-digit printed serial number may be acceptable. When defining the format of the printed serial number one should decide on whether and how to include the user-defined subfield. The hexadecimal format functions in any case; depending on the subfield content, one could take the digits as binary-coded decimal (BCD) and print them straight as decimal numbers. When printing the whole ROM registration number or parts of it in hexadecimal format, the CRC should be printed to the left and the family code to the right. Example: 25002066007A8081. Omitting family code and CRC, leaves 002066007A80, the customer identification and a combination of user-defined subfield and serial number. Knowing the ROM option (B in this example), one can tell that the actual serial number of this device is 007A80 in hexadecimal or 31360 decimal. In this example, the subfield can be seen as decimal as well as hexadecimal.
Generic Silicon, Nameplate Approach (References 2A, 2C, and 3A)
The preferred way to get a conventional serial number is to include it in decimal form in the nameplate. This type of serialization needs to be managed by the software that controls the board tester, a task that may not be as easy as expected. Since the serial number by definition needs to be different in every part, gang-programming the identification chips before they are mounted on the circuit board is not an option. Instead of including a serialization in the nameplate, one could fall back to the same procedure as in case
1) Generic Silicon, Minimalist approach.
An elegant way of solving the serialization problem, which arises with generic silicon and the nameplate approach, is UniqueWare. In contrast to ROM customization, with UniqueWare the customization takes place in the EPROM section of the chip. Starting with ROM-customized silicon, the EPROM is programmed with a project identification number, a customer-specified decimal or hexadecimal serialization, plus (optional) customer-supplied constant data. The factory-programmed memory pages are write-protected. The conventional serial number can be read straight from the EPROM and, if decimal serialization is chosen, be used without any number conversion.
All UniqueWare devices have a customized family code and share the customer ID 5E7 to distinguish them from generic silicon (see Table 4). The project identification number ensures that devices programmed for different customers/applications are always identifiable, even if the data specification is exactly the same. (See Table 6 for an example.) Since UniqueWare parts are only available programmed with customer-specific data—never as "blank" parts—they offer the same level of protection as other customized silicon parts, plus the benefits of a convenient serialization and a shorter lead time. For more information on UniqueWare and how to specify the desired data pattern and serialization refer to application note 99.
Table 4. UniqueWare ROM Section
Device Type
64-Bit Rom Registration Number
8-bit CRC
UniqueWare Identifier
Device Serial Number
Family Code
DS2502U
5
E
7
MSB (hex format) LSB
89
DS2505U
5
E
7
MSB (hex format) LSB
8B
DS2506U
5
E
7
MSB (hex format) LSB
8F
Example
D8
5
E
7
0 00 5D 43 0E
89
Connection to Main Circuit
To get the PnP data to the host CPU, the board identification chip must be connected to the circuitry on the board. The interface shown in Figure 2 fits to a bi-directional port. Due to its open-drain characteristic, port 0 of the 8051-compatible microcontroller is the preferred choice to drive a 1-Wire line.
Figure 2. Dual-Read Mode Interface.
During normal operation, the port pin is pulled high to VCC through the 2.2kΩ resistor. The parasitic diode inside the MOSFET ensures that the voltage at the 1-Wire device gets close to VCC even if VGS is zero and the MOSFET is off. If the microcontroller puts a logic 0 (0V) on the port pin, VGS is high enough to make the transistor conducting. This way the voltage at the port pin is propagated through the transistor to the 1-Wire device. To transmit a 0-bit to the microcontroller, the 1-Wire device shorts the 1-Wire line to Ground for a short time. During that time current flows from VCC through the resistor and through the parasite diode of the MOSFET through the 1-Wire device. Despite the voltage drop between source and drain of the MOSFET, the voltage on the port pin remains in the permissible range to safely be recognized as logic 0. To transmit a 1-bit, the 1-Wire device remains in high impedance. As a result, the 1-Wire line is pulled to VCC by the resistor as soon as the microcontroller stops pulling low. If the board is not powered, one can read the 1-Wire chip through the external probe point. The 2.2kΩ resistor ensures that VGS of the MOSFET is 0, which keeps the transistor non-conducting, preventing current flow from the external 1-Wire reader into the other circuitry on the board. The jumper shown in Figure 2 is only required with EPROM devices. It must be removed to protect the MOSFET from the 12V pulses, which the tester applies to program the EPROM chip with the nameplate data. After the programming is completed, the jumper must be reinstalled to give the microcontroller access to the identification chip.
The concept of the circuit in Figure 2 can be applied to other types of serial interface; however, a decoupling transistor will be necessary for the data line as well as clock and control signals. It may also be necessary to de-couple the power line of the serial memory chip. Instead of the simple 2-pin probe point, a 4- or 5-pin connector may be required. As a further disadvantage, non-1-Wire serial memory devices don't come with a built-in customizable serialization. With its security features, the DS2432 is unique in the market.
Real World Example
The TINI-board, which was chosen as an example for the nameplate design considerations, actually includes an electronic nameplate. Due to its Ethernet interface, each of these boards needs an Ethernet Address. The easiest way to implement such addresses in silicon is by means of UniqueWare. Since the UniqueWare data consumes only one memory page of the DS2502 memory chip, there were three memory pages available for the nameplate at no extra cost.
The actual TINI nameplate design is somewhat different from the sample design concept. This is acceptable as long as all relevant data is included, identifiable, and has a value. The TINI nameplate contains seven data entities or tags, as shown in Table 5.
Table 5. TINI Nameplate Contents
Tag Name
Tag #
Tag Length
Tag Format
Possible Tag Values
TAG_TINI_HW_VERSION
01
35 bytes
ASCII
(text string)
TAG_DATE_CODE
02
4 bytes
numeric
(UNIX 32-bit time format)
TAG_RAM_CHIP_SELECT_CONFIG
03
1 byte
numeric
1, 2
TAG_RAM_SIZE_EACH_CONFIG
04
3 bytes
numeric
0 to 16777216 bytes
TAG_SMC_NOT_PLACED
05
1 byte
numeric
0 = Ethernet available
1 = Ethernet unavailable
TAG_SMC_PORT_PINS_CONNECTED
06
1 byte
numeric
0 = pins not connected
1 = pins connected
TAG_NONVOLATIZER_NOT_PLACED
07
1 byte
numeric
0 = non-volatizer used
1 = non-volatizer not used
Each tag is identified by a number that is followed by a length byte and the tag value. Beginning with tag number 1, the seven tags are concatenated to form the TINI configuration data. The configuration data is reformatted into data packets of 32 bytes each, matching the size of a memory page. Each packet begins with a length byte and ends with a CRC16. The resulting data pattern including the UniqueWare data of a sample device is shown in Table 6.
Table 6. Sample TINI Identification Chip
UniqueWare Section
Address
0000
0A
29
11
00
00
B8
93
00
35
60
00
68
59
FF
FF
FF
0010
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
Nameplate Section
Address
0020
1D
01
23
44
53
20
54
49
4E
49
20
4D
6F
64
65
6C
0030
20
33
39
30
20
52
65
76
20
44
20
44
53
54
F3
3C
0040
1D
49
4E
49
31
2D
35
31
32
02
04
A0
3E
36
3A
03
0050
01
01
04
03
00
00
08
05
01
00
06
01
01
07
C5
01
0060
02
01
00
AF
AF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
0070
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
FF
Color Code Legend
Data Packet Length Byte
UniqueWare constant data (IEEE® company Identification)
Data Packet CRC
(no color = unused byte)
UniqueWare Project Identification Number
Nameplate Parameter Number and Parameter Length
UniqueWare serialization
Nameplate Parameter Value
To read the nameplate, one needs to identify the tag numbers that are followed by the tag size. As many subsequent bytes as indicated by the tag length are the tag value. The next byte again is a tag number, and so on. Table 7 shows the data of Table 6 in decoded form. When decoding the tag values, it is important to know that multi-byte numbers are stored with the least significant byte at the lower address ("Little Endian" byte order). To restore the natural sequence of digits one needs to write down the bytes (not digits) in reverse order (i.e., from right to left as shown in the "Explanation" column). Text is stored in its natural sequence (i.e., the first character at the lower address).
Ethernet company ID# 006035 (UniqueWare constant data)
Nameplate Section
01
(text)
Text = "DS TINI Model 390 Rev D DSTINI1-512"
02
A0 3E 36 3A
3A363EA0 = December 12, 2000, 15:05:04 hours (see note)
03
01
1 chip select
04
00 00 08
08 00 00 = decimal 524288 bytes (512K)
05
00
SMC Ethernet chip is available
06
01
SMC port pins are connected to SIMM edge connector
07
00
Non-volatizer is used
NOTE: The date code 3A363EA0 represents 976633504 seconds after 0 hours UTC of January 1st, 1970. This time format assumes that a day is exactly 24 hours. The conversion from seconds to time of day therefore is very straightforward. The computation of the date includes the leap year compensation and is more complex. For a description of the algorithm to convert from conventional time and date to seconds and vice versa see application note 517, "DS1371/DS1372/DS1374 32-Bit Binary Counter Time Conversion."
Since the configuration data is packetized using a length byte at the beginning and a CRC16 at the end, it is not possible to invalidate a parameter (e.g., to update the nameplate after a second memory chip is installed). This would have been possible without packetizing. This packetizing, however, protects the data in the chip against memory bit errors. The CRC16 would not match if a memory bit had flipped. To safeguard against bit errors during communication, all 1-Wire EPROM chips support a read function that automatically generates a CRC based on memory data and transmits it after the end of a memory page.
1-Wire is a registered trademark of Maxim Integrated Products, Inc.
IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers, Inc.
TINI is a registered trademark of Maxim Integrated Products, Inc.
UniqueWare is a trademark of Maxim Integrated Products, Inc.
Automatic Updates
Would you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EE-Mail™.
Download, PDF Format
