M48T59 ST Microelectronics, M48T59 Datasheet - Page 14

• Manufacturer Part Number: M48T59
• Description: 64 Kbit 8Kb x8 TIMEKEEPER SRAM
• Manufacturer: ST Microelectronics

M48T59, M48T59Y, M48T59V
The most significant bit of the Watchdog Register
is the Watchdog Steering Bit. When set to a ’0’, the
watchdog will activate the IRQ/FT pin when timed-
out. When WDS is set to a ’1’, the watchdog will
output a negative pulse on the RST pin for a dura-
tion of 40ms to 200ms. The Watchdog register and
the FT bit will reset to a ’0’ at the end of a watchdog
time-out when the WDS bit is set to a ’1’.
The watchdog timer resets when the microproces-
sor performs a re-write of the Watchdog Register.
The time-out period then starts over. The watch-
dog timer is disabled by writing a value of
00000000 to the eight bits in the Watchdog Regis-
ter. The watchdog function is automatically dis-
abled upon power-down and the Watchdog
Register is cleared. If the watchdog function is set
to output to the IRQ/FT pin and the frequency test
function is activated, the watchdog or alarm func-
tion prevails and the frequency test function is de-
nied.
BATTERY LOW FLAG
The M48T59/59Y/59V automatically performs pe-
riodic battery voltage monitoring upon power-up
and at factory-programmed time intervals of 24
hours (at day rollover) as long as the device is
powered and the oscillator is running. The Battery
Low flag (BL), Bit D4 of the flags Register 1FF0h,
will be asserted high if the internal or SNAPHAT
battery is found to be less than approximately
2.5V. The BL flag will remain active until comple-
tion of battery replacement and subsequent bat-
tery low monitoring tests, either during the next
Table 13. Default Values
Note: 1. WDS, BMB0-BMB4, RBO, RB1.
14/21
Initial Power-up
(Battery Attach for SNAPHAT)
Subsequent Power-up / RESET
Power-down
2. State of other control bits undefined.
3. State of other control bits remains unchanged.
4. Assuming these bits set to ’1’ prior to power-down.
(4)
Condition
(2)
(3)
W
0
0
0
R
0
0
0
power-up sequence or the next scheduled 24-hour
interval.
If a battery low is generated during a power-up se-
quence, this indicates that the battery voltage is
below 2.5V (approximately), which may be insuffi-
cient to maintain data integrity. Data should be
considered suspect and verified as correct. A fresh
battery should be installed.
If a battery low indication is generated during the
24-hour interval check, this indicates that the bat-
tery is near end of life. However, data has not been
compromised due to the fact that a nominal VCC
is supplied. In order to insure data integrity during
subsequent periods of battery back-up mode, it is
recommended that the battery be replaced. The
SNAPHAT top may be replaced while VCC is ap-
plied to the device.
Note: Battery monitoring is a useful technique only
when performed periodically. The M48T59/59Y/
59V only monitors the battery when a nominal
VCC 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.
POWER-ON DEFAULTS
Upon application of power to the device, the fol-
lowing register bits are set to a ’0’ state: WDS;
BMB0-BMB4; RB0-RB1; AFE; ABE; W; R; FT.
(See Table 13).
FT
0
0
0
AFE
0
0
1
ABE
0
0
1
WATCHDOG
Register
0
0
0
(1)