Maxim Integrated Secure Microcontroller - Manual (Page 160)

Secure Microcontroller User’s Guide
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17.3 Registers
The time information is contained in eight registers that are each 8 bits long. After the 64-bit recognition
pattern has been received, data in these registers is accessed one bit at a time that is shown conceptually
in
Figure 17-4 . It is recommended that data written to the RTC be handled in groups of 8 bits
corresponding to the register bytes in order to prevent erroneous results.
Register data is always in BCD format except for the hours register (register 3), whose format changes
depending upon the state of bit 7. If bit 7 is high, the 12-hour mode is selected and bit 5 of the hours
register becomes an AM/PM indicator; if bit 7 is low, the 24-hour mode is selected and bit 5 becomes the
second 10-hour bit (20–23 hours).
Figure 17-5 contains examples that illustrate the content of these
registers for different modes and times.
17.4 Special Bits
Bit 5 of the days register (register 4) is the control bit for the clock micropower oscillator. Clearing bit 5
to a logic 0 enables the oscillator for normal operation; setting bit 5 to a logic 1 disables the oscillator and
halts the timekeeping. It is recommended that bit 5 always be cleared to 0.
Register locations shown as logic 0s in
Figure 17-4 will always return a 0 when being read. Write
operations to these bit locations are ignored by the clock and have no effect on its operation.