AN925 Datasheet, PDF (2/4 Pages) STMicroelectronics – Time Update in ST’s TIMEKEEPER Devices

AN925 - APPLICATION NOTE
A 1 Hz clock signal, from the clock chain, is used to update the seconds register. Each rising edge of the
1 Hz clock signal increments the system side of the seconds register. Having updated the seconds register
a ripple carry to other registers might be initiated (for example, incrementing the minutes register from 00
to 01, after the seconds register has been incremented from 59 to 00). The longest possible ripple carry
extends through all seven registers: seconds, minutes, hours, day of the week, date of the month, month
of the year and year.
Figure 2 shows two consecutive updates of the seconds register. The first update only updates the sec-
onds register; the second update, though, ripples through all seven clock registers. When the system-side
time registers have finished being updated, they are copied across to the user-side, thereby making the
updated time available to the user. Thus, the spacing between successive System-to-User-Update-Pulse
is one second plus a delta delay that can vary from 0.5 ms to 3.5 ms (1x0.5 ms to 7x0.5 ms). The M48T58
(revision B), M48T59 (revision B), M48T35 and M48T559 are examples of TIMEKEEPER devices that op-
erate in this way.
Figure 2. Time Update Waveform Diagram (variable delay)
1 Hz clock frequency
Seconds register update
Minutes register update
Hours register update
Day of week register update
Date register update
Month register update
Year register update
Reference update pulse
System to user update pulse
1 second + 3.5ms
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