Abstract: The DS1340-33 Serial Access Real-Time Clock (RTC) provides a clock/calendar that is pin-compatible and functionally equivalent to the ST Microelectronics M41T00, including the software clock calibration function.
If used as a drop-in replacement, no PCB or software changes are required. In fact, the DS1340-33 is an enhancement because it provides a lower timekeeping voltage (1.3V compared to 2.0V), 2-wire operation down to 1.8V, 2-wire bus speeds up to 400kHz, and a smaller package option. It also provides trickle charge capability on the VBACKUP pin and an oscillator stop flag bit, which requires two additional registers.
Package
The DS1340 provides two packaging options. The first option is an 8-pin SO (150-mils wide), which is identical to the M41T00M6. This would be the package selected if the DS1340 is being used as a second-source device. The second option is a smaller 8-pin µSOP. This package's dimensions are detailed in the following link: http://pdfserv.maxim-ic.com/arpdf/Packages/g2018_001.pdf.
Electrical Characteristics
The following chart details the electrical characteristics for the DS1340-33 and the M41T00.
Parameter
DS1340-33
M41T00
Units
Min
Typ
Max
Min
Typ
Max
Supply Voltage
2.97
3.3
5.5
2.0
5.5
V
Backup Supply Voltage
1.3
5.5
2.0
3.0
3.5
V
Power-Fail Voltage
2.70
2.8
2.97
VBAT - 0.70
VBAT - 0.50
VBAT - 0.20
V
Operating Current
120
200
300
µA
Standby Current
85
125
70
µA
Timekeeping Current, 2.97 to 5.5V, -40°C to +85°C
0.85
1.25
Not specified
Not specified
µA
Timekeeping Current, 3.3V, +25°C
0.8
1.0
0.8
1.0
µA
By examining the table, it becomes obvious that the DS1340-33 provides several enhancements over the M41T00.
The DS1340-33 supply voltage range is smaller, but lower operating voltages are available for the DS1340 with the -3 and -18 options. Overall, the DS1340 provides three operating ranges of 1.71V to 1.89V, 2.7V to 3.3V, and 2.97V to 5.5V.
One of the major advantages of the DS1340-33 is a much wider backup supply-voltage range. Timekeeping remains valid all the way down to 1.3V. When using a super cap with the integrated trickle charger, the backup supply can be charged to the supply voltage.
The DS1340-33 provides a fixed trip point, which is the voltage where power is switched between the supply voltage and the back-up supply. The M41T00 trip point is dependent on the backup supply voltage.
The typical timekeeping current values for the DS1340-33 and M41T00, as specified at 3.3V and +25°C, are identical. Dallas Semiconductor has verified that the DS1340-33 timekeeping current is less sensitive to voltage and temperature than the M41T00. As a result, the user can expect the DS1340-33 timekeeping current to be lower over the entire operating voltage and temperature ranges.
Register Map
The register map of the DS1340 is identical to the M41T00 so no firmware changes are required for existing sockets. Two additional registers are required for the trickle charger and status (shaded). These are designed to be transparent to any system that does not use them.
Table 1. Register Map
Address
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Function
Range
00H
EOSC
10 Seconds
Seconds
Seconds
00-59
01H
R/W
10 Minutes
Minutes
Minutes
00-59
02H
CEB
CB
10 Hours
Hours
Century/ Hours
0-1; 00-23
03H
R/W
R/W
R/W
R/W
R/W
Day
Day
01-07
04H
R/W
R/W
10 Date
Date
Date
00-31
05H
R/W
R/W
R/W
10 Month
Month
Month
01-12
06H
10 Year
Year
Year
00-99
07H
OUT
FT
S
CAL4
CAL3
CAL2
CAL1
CAL0
Control
08H
TCS3
TCS2
TCS1
TCS0
DS1
DS0
ROUT1
ROUT0
Trickle Charger
09H
OSF
0
0
0
0
0
0
0
Flag
For more detail on each register, refer to the DS1340 data sheet.
Block access of the register map is identical to the ST device. During a multibyte access of the timekeeping registers, when the address pointer reaches 07h, the end of the RTC and control register space, it wraps around to location 00h.
The additional registers at address locations 08h and 09h are accessed by writing the address pointer to the corresponding location. In block mode, the address sequence is 08h, 09h, and then the address pointer wraps around to location 00h. The sequence then is identical to the M41T00.
If firmware is written to use either RTC, the additional registers can be used to identify the device in use. The M41T00 aliases the register set every eight addresses. That is, Register 0 can be accessed at addresses 0H, 8H, 10H, 18H,... The DS1340-33 does not repeat this pattern because it has unique registers at address 8H and 9H. A flow chart for identifying the RTC being used is included at the end of this application note.
2-Wire Interface
The DS1340-33 supports both standard and fast modes of the 2-wire interface. This means the SCL clock frequency can go to a maximum of 400kHz. The M41T00 only supports the standard mode, which has a maximum frequency of 100kHz.
Summary
As demonstrated, the DS1340-33 is not only an adequate second source for the ST M41T00, but it provides many improvements. These include a wider operating range for the primary and backup supplies, a smaller package, and a faster interface speed. In addition it also provides an integrated trickle charger and a flag that indicates when the oscillator may have stopped.
Flowchart for Identifying RTC
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
