 Two
serial interface standards for peripheral communications dominate
the market:
• I2C (Inter-Integrated Circuit); and
• SPI (Serial Peripheral Interface).
While the I2C interface is very popular and requires
only two pins, the four-wire SPI bus has grown
in popularity and can operate at much higher speeds than I2C
with full duplex transfers of 25MHz and higher, greatly exceeding
the 400Kbit/s, half-duplex performance of I2C.
Plus, when there is no serial interface on the microcontroller,
users are much more comfortable emulating SPI in software (“bit
banging”) than I2C. Since ST is a market leader
in Serial RTCs it is only logical that they would offer SPI-compatible
RTCs. |
| M41T94: ST's First SPI-based Serial RTC |
| The M41T94 is best described as a combination between a Serial
RTC and a Microprocessor Supervisor. |
Serial RTC Features include:
 |
Counters for Tenths/Hundredths of Seconds, Seconds,
Minutes, Hours, Day, Date, Month, Year, and Century |
|
Programmable Alarm with Repeat Modes |
|
44 Bytes of general-purpose NVRAM |
|
Programmable Square Wave Generator - 1Hz to 32KHz |
|
Automatic Battery Switchover and Write Protect Circuits |
Microprocessor Supervisor Features include:
|
Power-on Reset/Low-Voltage Detect |
|
Programmable Watchdog - 62.5ms to 128s |
|
Two Reset Inputs |
|
 |
|
| Other Features include: |
|
| M41ST95: ST's SPI Bus "Non-Volatizor"
Microprocessor |
The M41ST95 builds on the M41T94 and adds several features,
including NVRAM Supervisor functionality with a Vout pin and Chip
Enable Gate. This allows the RTC’s battery to also back
up an external low power SRAM.
The Early Power-Fail Warning can be used to monitor the power
supply upstream and provide advanced warning of power failure
thereby allowing the system to shutdown smoothly.
A dedicated 32kHz output is available for systems which employ
phase-locked loop clock synthesis and need a 32kHz reference.
This output defaults to ON, but can be turned off when it is not
needed. |
| Serial RTC Features include: |
 |
|
| Microprocessor Supervisor Features include: |
|
Power-on Reset/Low-Voltage Detect |
|
Programmable Watchdog - 62.5ms to 128s |
|
Power-fail In/Power-fail Out (PFI/PFO) with 1.25V Precision
Reference. Provides Early Power-fail Warning |
|
Two Reset Inputs |
|
| NVRAM Supervisor Features include: |
|
Automatic Battery Switchover and Write Protect; non-volatizes
the external LPSRAM |
|
Battery Monitor |
|
| Other Features include: |
|
| More Information |
|
