AN925 - APPLICATION NOTE

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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

12/31/96 23:59:59

1/1/97 00:00:00

AI02483