Datasheet

Clock operation M41ST87Y, M41ST87W
40/54 Doc ID 9497 Rev 10
The M41ST87Y/W only monitors the battery when a nominal V
CC
is applied to the device.
Thus applications which require extensive durations in the battery back-up mode should be
powered-up periodically (at least once every few months) in order for this technique to be
beneficial. Additionally, if a battery low is indicated, data integrity should be verified upon
power-up via a checksum or other technique.
3.14 t
rec
bit
Bit D7 of clock register 04h contains the t
rec
bit (TR). t
rec
refers to the automatic continuation
of the deselect time after V
CC
reaches V
PFD
. This allows for a voltage settling time before
WRITEs may again be performed to the device after a power-down condition. The t
rec
bit will
allow the user to set the length of this deselect time as defined by Table 12.
Table 12. t
rec
definitions
3.15 Electronic serial number
The M41ST87Y/W has a unique 8-byte lasered serial number with parity. This serial number
is “read only” and is generated such that no two devices will contain an identical number.
3.16 Oscillator stop detection
If the oscillator fail (OF) bit is internally set to a '1,' this indicates that the oscillator has either
stopped, or was stopped for some period of time, and can be used to judge the validity of the
clock and date data. This bit will be set to '1' any time the oscillator stops. The following
conditions can cause the OF bit to be set:
● The first time power is applied (defaults to a '1' on power-up).
● The voltage present on V
CC
or battery is insufficient to support oscillation.
● The ST bit is set to '1.'
If the oscillator fail interrupt enable bit (OFIE) is set to a '1,' the IRQ
/OUT pin will also be
asserted. The IRQ
/OUT output is cleared by resetting the OF bit to '0,' resetting the OFIE bit
to '0,' or if the RST
output is asserted (but is NOT cleared by reading the flag register).
The OF bit will remain set to '1' until written to logic '0.' The oscillator must start and have
run for at least 4 seconds before attempting to reset the OF bit to '0.' This function operates
both under normal power and in battery backup. If the trigger event occurs during a power-
down condition, this bit will be set correctly.
Note: The ABE bit must be set to '1' for the IRQ
/OUT pin to be activated in battery backup.
t
rec
bit (TR) STOP bit (ST)
t
rec
time
Units
Min Max
0 0 96 98
(1)
1. Default setting.
ms
0140200ms
1 X 50 2000 µs