December 1998 1/4
AN925
APPLICATION NOTE
Time Update in ST’s TIMEKEEPER Devices
Figure 1 shows how the non-volatile, static memory array and the quartz controlled clock oscillator, of
TIMEKEEPER devices from STMicroelectronics, are interconnected through the clock registers. The clock
registers are mapped into the memory array (please see the data sheet for the precise mapping) as 8 or
16 BYTEWIDE BIPORT memory cells. The time data in these memory cells are updated from the clock
side (the system side) and are made available to the user side within the user’s finest time resolution.
However, the user’s finest time resolution is one second, so this leaves plenty of scope for variability (of
the order of several milliseconds) between one update and the next. Since this variability might be notice-
able to some applications (for example, those that poll the time registers regularly, or those that use an
alarm function that is triggered once per second), this document sets out to explain the nature of the var-
iability, to make it more predictable to the applications designer.
Figure 1. Internal Architecture of an ST TIMEKEEPER Device
AI02482
LITHIUM
CELL
OSCILLATOR AND
CLOCK CHAIN
V
PFD
V
CC
V
SS
32,768 Hz
CRYSTAL
VOLTAGE SENSE
AND
SWITCHING
CIRCUITRY
8, 16 x 8
TIMEKEEPER
REGISTERS
SRAM ARRAY
A0-AX
DQ0-DQ7
POWER
WDI
E
W
G
BATTERY LOW
IRQ/FT RST
INTEGRATED BATTERY
CRYSTAL AND
SNAPHAT