Abstract: A microprocessor can easily generate 1-Wire® timing signals if a true bus master is not present (e.g., DS2480B, DS2490). This application note provides an example, written in 'C', of the basic standard speed 1-Wire master communication routines. The four basic operations of a 1-Wire bus are Reset, Write 1 bit, Write 0 bit, and Read bit. Byte functions can then be derived from multiple calls to the bit operations. The time values provided produce the most robust 1-Wire master for communication with all 1-Wire devices over various line conditions.
Introduction
A microprocessor can easily generate 1-Wire timing signals if a dedicated bus master is not present. This application note provides an example, written in 'C', of the basic standard speed 1-Wire master communication routines. Overdrive communication speed is also covered by this document. There are several system requirements for proper operation of the code examples:
The communication port must be bidirectional, its output is open-drain, and there is a weak pull-up on the line. This is a requirement of any 1-Wire bus. See Appendix A in Guidelines for Reliable 1-Wire Networks (Application Note 148) for a simple example of a 1-Wire master microprocessor circuit.
The system must be capable of generating an accurate and repeatable 1µs delay for standard speed and 0.25µs delay for overdrive speed.
The communication operations must not be interrupted while being generated.
The four basic operations of a 1-Wire bus are Reset, Write 1 bit, Write 0 bit, and Read bit. The time it takes to perform one bit of communication is called a time slot in the device datasheets. Byte functions can then be derived from multiple calls to the bit operations. See Table 1 below for a brief description of each operation and a list of the steps necessary to generate it. Figure 1 illustrates the waveforms graphically. Table 2 shows the minimum, maximum and recommended timings for the 1-Wire master to communicate with 1-Wire devices over the most common line conditions. Alternate minimum and maximum values can be used when restricting the 1-Wire master to a particular set of devices and line conditions.
Table 1. 1-Wire Operations
Operation
Description
Implementation
Write 1 bit
Send a '1' bit to the 1-Wire slaves (Write 1 time slot)
Drive bus low, delay A Release bus, delay B
Write 0 bit
send a '0' bit to the 1-Wire slaves (Write 0 time slot)
Drive bus low, delay C Release bus, delay D
Read bit
Read a bit from the 1-Wire slaves (Read time slot)
Drive bus low, delay A
Release bus, delay E
Sample bus to read bit from slave
Delay F
Reset
Reset the 1-Wire bus slave devices and ready them for a command
Delay G
Drive bus low, delay H
Release bus, delay I
Sample bus, 0 = device(s) present, 1 = no device present
Delay J
Expressed as tW1L (write 1 low) minus ε (rise time to VTH) plus tF (fall time to VTL) in the device datasheets.
Assume a mid-range standard speed network with a rise and fall time of less then 3µs.
Assume a small overdrive speed network with a rise and fall time less then 0.5µs.
Expressed as tSLOT (time slot length) in the device datasheets minus the value used for 'A'.
Expressed as tW0L (write 0 low) minus δ (rise time to VIHMASTER) plus tF (fall time to VTL) in the device datasheets.
Expressed as tREC (recovery time) plus δ (rise time to VIHMASTER) in the device datasheets.
Expressed as tMSR (master sample read) plus tF (fall time to VTL) in the device datasheets minus the value used for 'A'.
Sample as late as possible in the range to provide the most rise time recovery.
Expressed as tSLOT (time slot length) in the device datasheets minus the value used for 'A' minus the value used for 'E'.
Expressed as tRSTL (reset low time) minus ε (rise time to VTH) plus tF (fall time to VTL) in the device datasheets.
Expressed as tMSP (master sample presence) plus ε (rise time to VTH) in the device datasheets.
Min expressed as tRSTL (reset low time) in the device datasheets minus the value used for 'I'.
1-Wire reset operation described here does not take into account the extended (alarming) presence pulse sequence that can be produced by the DS2404 and DS1994 devices. See the device data sheets for this special case. The end of the 1-Wire reset sequence can be sampled to verify that the 1-Wire bus has returned to the pull-up level. If the level is still 0 then the 1-Wire bus may be shorted to ground or an alarming DS2404/DS1994 may be present.
Expressed as tREC (recovery time) minus the value used for 'D'. Some devices in overdrive require an extra delay before a tRSTL (reset low time) to make sure that the device's parasite power supply is fully charged.
For low-voltage operation, some devices need a longer value. See device datasheet for the applicable value.
Code Examples
This following code samples rely on two common 'C' functions outp and inp to write and read bytes of data to IO port locations. They are typically located in the <conio.h> standard library. These functions can be replaced by platform appropriate functions.
// send 'databyte' to 'port'
int outp(unsigned port, int databyte);
// read byte from 'port'
int inp(unsigned port);
The constant PORTADDRESS in the code (see Figure 3) is defined as the location of the communication port. The code assumes bit 0 of this location controls the 1-Wire bus. Setting this bit to 0 will drive the 1-Wire line low. Setting this bit to 1 will release the 1-Wire to be pulled up by the resistor pull-up or pulled-down by a 1-Wire slave device.
The function tickDelay in the code is a user-generated routine to wait a variable number of 1/4 microseconds. This function will vary for each unique hardware platform running so it is not implemented here. Below is the function declaration for the tickDelay along with a function SetSpeed to set the recommended standard and overdrive speed tick values.
Example 1. 1-Wire Timing Generation
// Pause for exactly 'tick' number of ticks = 0.25us
void tickDelay(int tick); // Implementation is platform specific
// 'tick' values
int A,B,C,D,E,F,G,H,I,J;
//-----------------------------------------------------------------------------
// Set the 1-Wire timing to 'standard' (standard=1) or 'overdrive' (standard=0).
//
void SetSpeed(int standard)
{
// Adjust tick values depending on speed
if (standard)
{
// Standard Speed
A = 6 * 4;
B = 64 * 4;
C = 60 * 4;
D = 10 * 4;
E = 9 * 4;
F = 55 * 4;
G = 0;
H = 480 * 4;
I = 70 * 4;
J = 410 * 4;
}
else
{
// Overdrive Speed
A = 1.5 * 4;
B = 7.5 * 4;
C = 7.5 * 4;
D = 2.5 * 4;
E = 0.75 * 4;
F = 7 * 4;
G = 2.5 * 4;
H = 70 * 4;
I = 8.5 * 4;
J = 40 * 4;
}
}
Example 2 below shows the code examples for the basic 1-Wire operations.
Example 2. 1-Wire Basic Functions
//-----------------------------------------------------------------------------
// Generate a 1-Wire reset, return 1 if no presence detect was found,
// return 0 otherwise.
// (NOTE: Does not handle alarm presence from DS2404/DS1994)
//
int OWTouchReset(void)
{
int result;
tickDelay(G);
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(H);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(I);
result = inp(PORTADDRESS) & 0x01; // Sample for presence pulse from slave
tickDelay(J); // Complete the reset sequence recovery
return result; // Return sample presence pulse result
}
//-----------------------------------------------------------------------------
// Send a 1-Wire write bit. Provide 10us recovery time.
//
void OWWriteBit(int bit)
{
if (bit)
{
// Write '1' bit
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(A);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(B); // Complete the time slot and 10us recovery
}
else
{
// Write '0' bit
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(C);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(D);
}
}
//-----------------------------------------------------------------------------
// Read a bit from the 1-Wire bus and return it. Provide 10us recovery time.
//
int OWReadBit(void)
{
int result;
outp(PORTADDRESS,0x00); // Drives DQ low
tickDelay(A);
outp(PORTADDRESS,0x01); // Releases the bus
tickDelay(E);
result = inp(PORTADDRESS) & 0x01; // Sample the bit value from the slave
tickDelay(F); // Complete the time slot and 10us recovery
return result;
}
This is all for bit-wise manipulation of the 1-Wire bus. The above routines can be built upon to create byte-wise manipulator functions as seen in Example 3.
Example 3. Derived 1-Wire Functions
//-----------------------------------------------------------------------------
// Write 1-Wire data byte
//
void OWWriteByte(int data)
{
int loop;
// Loop to write each bit in the byte, LS-bit first
for (loop = 0; loop < 8; loop++)
{
OWWriteBit(data & 0x01);
// shift the data byte for the next bit
data >>= 1;
}
}
//-----------------------------------------------------------------------------
// Read 1-Wire data byte and return it
//
int OWReadByte(void)
{
int loop, result=0;
for (loop = 0; loop < 8; loop++)
{
// shift the result to get it ready for the next bit
result >>= 1;
// if result is one, then set MS bit
if (OWReadBit())
result |= 0x80;
}
return result;
}
//-----------------------------------------------------------------------------
// Write a 1-Wire data byte and return the sampled result.
//
int OWTouchByte(int data)
{
int loop, result=0;
for (loop = 0; loop < 8; loop++)
{
// shift the result to get it ready for the next bit
result >>= 1;
// If sending a '1' then read a bit else write a '0'
if (data & 0x01)
{
if (OWReadBit())
result |= 0x80;
}
else
OWWriteBit(0);
// shift the data byte for the next bit
data >>= 1;
}
return result;
}
//-----------------------------------------------------------------------------
// Write a block 1-Wire data bytes and return the sampled result in the same
// buffer.
//
void OWBlock(unsigned char *data, int data_len)
{
int loop;
for (loop = 0; loop < data_len; loop++)
{
data[loop] = OWTouchByte(data[loop]);
}
}
//-----------------------------------------------------------------------------
// Set all devices on 1-Wire to overdrive speed. Return '1' if at least one
// overdrive capable device is detected.
//
int OWOverdriveSkip(unsigned char *data, int data_len)
{
// set the speed to 'standard'
SetSpeed(1);
// reset all devices
if (OWTouchReset()) // Reset the 1-Wire bus
return 0; // Return if no devices found
// overdrive skip command
OWWriteByte(0x3C);
// set the speed to 'overdrive'
SetSpeed(0);
// do a 1-Wire reset in 'overdrive' and return presence result
return OWTouchReset();
}
The owTouchByte operation is a simultaneous write and read from the 1-Wire bus. This function was derived so that a block of both writes and reads could be constructed. This is more efficient on some platforms and is commonly used in API's provided by Dallas Semiconductor. The OWBlock function simply sends and receives a block of data to the 1-Wire using the OWTouchByte function. Note that OWTouchByte(0xFF) is equivalent to OWReadByte() and OWTouchByte(data) is equivalent to OWWriteByte(data).
These functions plus tickDelay are all that are required for basic control of the 1-Wire bus at the bit, byte, and block level. The following example in Example 4 shows how these functions can be used together to read a SHA-1 authenticated page of the DS2432.
Example 4. Read DS2432 Example
//-----------------------------------------------------------------------------
// Read and return the page data and SHA-1 message authentication code (MAC)
// from a DS2432.
//
int ReadPageMAC(int page, unsigned char *page_data, unsigned char *mac)
{
int i;
unsigned short data_crc16, mac_crc16;
// set the speed to 'standard'
SetSpeed(1);
// select the device
if (OWTouchReset()) // Reset the 1-Wire bus
return 0; // Return if no devices found
OWWriteByte(0xCC); // Send Skip ROM command to select single device
// read the page
OWWriteByte(0xA5); // Read Authentication command
OWWriteByte((page << 5) & 0xFF); // TA1
OWWriteByte(0); // TA2 (always zero for DS2432)
// read the page data
for (i = 0; i < 32; i++)
page_data[i] = OWReadByte();
OWWriteByte(0xFF);
// read the CRC16 of command, address, and data
data_crc16 = OWReadByte();
data_crc16 |= (OWReadByte() << 8);
// delay 2ms for the device MAC computation
// read the MAC
for (i = 0; i < 20; i++)
mac[i] = OWReadByte();
// read CRC16 of the MAC
mac_crc16 = OWReadByte();
mac_crc16 |= (OWReadByte() << 8);
// check CRC16...
return 1;
}
Additional Software
The basic 1-Wire functions provided in this application note can be used as a foundation to build sophisticated 1-Wire applications. One important operation omitted in this document is the 1-Wire search. The search is a method to discover the unique ID's of multiple 1-Wire slaves connected to the bus. Application Note 187: 1-Wire Search Algorithm describes this method in detail and provides 'C' code that can be used with these basic 1-Wire functions.
If a software solution is not feasible for a specific application, then a 1-Wire master chip or a synthesized 1-Wire master block can be used as an alternative.
Dallas Semiconductor provides a predefined 1-Wire master in Verilog and VHDL. DS1WM
There are several 1-Wire master chips that can be used as a peripheral to a microprocessor. The DS2480B Serial 1-Wire Line Driver provides easy connectivity to a standard serial port.
Revision History
07/06/00: Version 1.0—Initial release.
05/28/02: Version 2.0—Correct 1-Wire reset sample time. Add wave figure, links, and more code examples.
02/02/04: Version 2.1—Add overdrive support, provided min/max on timings, and update example.
09/06/05: Version 2.2—Correct polarity of PIO in Code Examples description.
1-Wire is a registered trademark of Maxim Integrated Products, Inc.
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