M41T00 Datasheet, PDF (8/15 Pages) STMicroelectronics – Serial Access TIMEKEEPER

M41T00
Figure 6. Serial Bus Data Transfer Sequence
CLOCK
DATA LINE
STABLE
DATA VALID
DATA
START
CONDITION
CHANGE OF
DATA ALLOWED
Figure 7. Acknowledgment Sequence
START
SCLK FROM
MASTER
1
2
DATA OUTPUT
BY TRANSMITTER
MSB
DATA OUTPUT
BY RECEIVER
STOP
CONDITION
AI00587
CLOCK PULSE FOR
ACKNOWLEDGEMENT
8
9
LSB
AI00601
CLOCK OPERATION
The eight byte clock register (see Table 3) is used
to both set the clock and to read the date and time
from the clock, in a binary coded decimal format.
Seconds, Minutes, and Hours are contained within
the first three registers. Bits D6 and D7 of clock
register 2 (Hours Register) contain the CENTURY
ENABLE Bit (CEB) and the CENTURY Bit (CB).
Setting CEB to a ’1’ will cause CB to toggle, either
from ’0’ to ’1’ or from ’1’ to ’0’ at the turn of the cen-
tury (depending upon its initial state). If CEB is set
to a ’0’, CB will not toggle. Bits D0 through D2 of
register 3 contain the Day (day of week). Registers
4, 5 and 6 contain the Date (day of month), Month
and Years. The final register is the Control Regis-
ter (this is described in the Clock Calibration sec-
tion). Bit D7 of register 0 contains the STOP Bit
(ST). Setting this bit to a ’1’ will cause the oscillator
to stop. If the device is expected to spend a signif-
icant amount of time on the shelf, the oscillator
may be stopped to reduce current drain. When re-
set to a ’0’ the oscillator restarts within one second.
The seven Clock Registers may be read one byte
at a time, or in a sequential block. The Control
Register (Address location 7) may be accessed in-
dependently. Provision has been made to assure
that a clock update does not occur while any of the
seven clock addresses are being read. If a clock
address is being read, an update of the clock reg-
isters will be delayed by 250ms to allow the read
to be completed before the update occurs. This
will prevent a transition of data during the read.
Note: This 250ms delay affects only the clock reg-
ister update and does not alter the actual clock
time.
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