Real-time clock

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There are various Real-time clock (RTC) chips used within the TI and Geneve systems.

MM58274 (Geneve)

The left address column shows the address mapping in the Native mode, while the right address column shows the GPL mode mapping.

F130 / 8010 Control register
F131 / 8011 Tenths of seconds
F132 / 8012 Units of seconds
F133 / 8013 Tens of seconds
F134 / 8014 Units of minutes
F135 / 8015 Tens of minutes
F136 / 8016 Units of hours
F137 / 8017 Tens of hours
F138 / 8018 Units of days
F139 / 8019 Tens of days
F13A / 801A Units of months
F13B / 801B Tens of months
F13C / 801C Units of years
F13D / 801D Tens of years
F13E / 801E Day of week
F13F / 801F Clock setting / Interrupt registers

There are some points to consider for both reading and writing.


Reading from a clock register is done by doing a MOVB from the respective address. Since the Geneve can address single bytes, only the addressed byte will be read.

However, the clock is only connected by 4 lines to the 8-bit data bus. The leading 4 bits (the high nibble) take some other value. We can observe:

  • In Native mode, the high nibble is 0xF (binary 1111)
  • In GPL mode, the high nibble is 0x1 (0001) for 8010-801E, and 0x2 (0010) for 801F.

Therefore, the read byte should be masked with 0x0F. The reason for this behavior is not known; maybe the high four bits are delivered by stale values in the Gate array.


Setting the clock registers is done via MOVB to the address. However, this may not show the expected effect. According to the datasheet of the MM58274, a certain procedure must be followed to properly set the clock.