Datasheet

Clock operation M41ST87Y, M41ST87W
38/54 Doc ID 9497 Rev 10
Table 10. Reset AC characteristics
3.9 Power-fail comparators (1 and 2)
Two power-fail inputs (PFI
1
and PFI
2
) are compared to an internal reference voltage (1.25V).
If either PFI
1
or PFI
2
is less than the power-fail threshold (V
PFI
), the associated power-fail
output (PFO
1
or PFO
2
) will go low. This function is intended for use as an under-voltage
detector to signal a failing power supply. Typically PFI
1
and PFI
2
are connected through
external voltage dividers (see Figure 5 on page 11) to either the unregulated DC input (if it is
available) or the regulated output of the V
CC
regulator. The voltage divider can be set up
such that the voltage at PFI
1
or PFI
2
falls below V
PFI
several milliseconds before the
regulated V
CC
input to the M41ST87Y/W or the microprocessor drops below the minimum
operating voltage, thus providing an early warning of power failure.
During battery back-up, the power-fail comparator turns off and PFO
1
and PFO
2
go (or
remain) low. This occurs after V
CC
drops below V
PFD
(min). When power returns, PFO
1
and
PFO
2
are forced high, irrespective of V
PFI
for the write protect time (t
rec
), which is the time
from V
PFD
(max) until the inputs are recognized. At the end of this time, the power-fail
comparator is enabled and PFO
1
and PFO
2
follow PFI
1
and PFI
2
. If the comparator is
unused, PFI
1
or PFI
2
should be connected to V
SS
and the associated PFO
1
or PFO
2
left
unconnected.
3.10 Power-fail outputs
The PFO
1
and PFO
2
outputs are programmable as N-channel, open drain output drivers, or
full-CMOS output drivers. By setting the power-fail output open drain bit (PFOD) to a '1,' the
output will be configured as open drain (with I
OL
as specified in Table 17 on page 44). When
PFOD is set to '0,' the outputs will be configured as full-CMOS (sink and source current as
specified in Table 17 on page 23).
Note: When configured as open drain (PFOD = '1'), PFO
1
and PFO
2
will require an external pull-
up resistor.
Symbol Parameter
(1)
1. Valid for ambient operating temperature: T
A
= –40 to 85 °C; V
CC
= 4.5 to 5.5 V or 2.7 to 3.6 V (except
where noted).
Min Max Unit
t
R1
(2)
2. Pulse widths of less than 100 ns will result in no RESET (for noise immunity).
RSTIN1 low to RST low (min pulse width) 100 200 ns
t
R2
(2)
RSTIN2 low to RSTIN2 high (min pulse width) 100 200 ns
t
rec
(3)
3. Programmable (see Table 12 on page 40). Same function as power-on reset.
RSTIN1 or RSTIN2 high to RST high 96 98
(3)
ms