TN0440 STMICROELECTRONICS | Alldatasheet
Document overview
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Technical content
various serial clock frequency values. the 1 MHz and 400 kHz I²C-bus specification. Table 1. Applicable products
1 M24xxx EEPROM: 400 kHz and 1 MHz clock
1.1 M24xxx 400 kHz AC tables until 2007
- January 2000: the I²C-bus specification defines a clock rate of either 100 kHz, 400 kHz or 3.4 MHz(a).
- June 2007: the I²C-bus specification defines a new table offering all timings related to the 1 MHz clock. Before 2007, the M24xxx EEPROM devices offered by ST were specified with an AC table based on a 400 kHz (max) clock rate. Two main differences appear when comparing the 400 kHz M24xxx AC table with the I²C- bus specification AC table (see Table 2):
- the min value of Data out hold time (tCLQX);
- the max value of Pulse width ignored (tNS). Why such differences? The M24xxx devices are designed to offer the best performances compatible with the I²C- bus specification, that is:
- tCLQX The min value of tCLQX provides a delay for the data out time (from the falling edge of the serial clock SCL: time for the data line SDA to hold the previous bit value before changing state). This min value pushes away the SDA change from the SCL falling edge: that is a safe timing condition, as SDA and SCL must never switch at the same time (such an event is a critical event for the I²C-bus, as it could be randomly decoded as either a Start, a Stop or a Data-in event).
- t NS The greater the value of tNS, the better the filter can screen out spikes added on the I²C bus. a. The 3.4 MHz clock rate is not supported by the M24xxx EEPROM.
Table 2. Access time and Noise filter parameters specified in the 400 kHz AC table
1.2 M24xxx 1 MHz AC tables after 2007
I²C-bus specification were designed with timings able to offer the best performances.
- tCLQX(min) is here again specified with a positive value (as this offers a safe timing condition, as explained for the previous 400 kHz table).
- tNS is now equal or better than the value defined in the I²C-bus specification.
1.3 Compatibility of 400 kHz and 1 MHz AC timing specifications
- all M24xxx products specified at 1 MHz can run at lower frequencies, and therefore can run at 400 kHz ;
- old products designed only for the 400 kHz AC tables (AC tables before 2007) cannot run at 1 MHz.
Table 3. Access time and Noise filter parameters specified in the 1 MHz AC table
any application running at 1 MHz). Table 4. I²C EEPROM and max operating clock frequency
2 Revision history
Table 5. Document revision history 17-Dec-2010 1 Initial release.