Datasheet

M41T56 Clock operation
Doc ID 6104 Rev 9 13/27
3 Clock operation
The eight byte clock register (see Ta bl e 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 century (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 register (this is described in the clock calibration
section). 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 significant amount of time on the shelf, the oscillator
may be stopped to reduce current drain. When reset 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 independently. 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 registers will be
delayed by 250 ms to allow the READ to be completed before the update occurs. This will
prevent a transition of data during the READ.
Note: This 250 ms delay affects only the clock register update and does not alter the actual clock
time.
Table 3. Register map
(1)
1. Keys:
S = Sign bit
FT = Frequency test bit
ST = Stop bit
OUT = Output level
X = Don't care
CEB = Century enable bit
CB = Century bit
Address
Data
Function/range
BCD format
D7 D6 D5 D4 D3 D2 D1 D0
0 ST 10 Seconds Seconds Seconds 00-59
1 X 10 Minutes Minutes Minutes 00-59
2CEB
(2)
2. When CEB is set to '1,' CB toggles from '0' to '1' or from '1' to '0' every 100 years (dependent upon the
initial value set). When CEB is set to '0,' CB does not toggle.
CB 10 hours Hours Century/hours 0-1/00-23
3XXXXX Day Day01-07
4 X X 10 date Date Date 01-31
5 X X X 10 M. Month Month 01-12
6 10 years Years Year 00-99
7 OUT FT S Calibration Control