BQ4845_14 Datasheet, PDF (6/24 Pages) Texas Instruments – Parallel RTC With CPU Supervisor

bq4845/bq4845Y
This activity unconditionally write-protects the external
SRAM as VCC falls below VPFD. If a memory access is in
progress to the external SRAM during power-fail detec-
tion, that memory cycle continues to completion before
the memory is write-protected. If the memory cycle is
not terminated within time tWPT, the chip enable output
is unconditionally driven high, write-protecting the con-
trolled SRAM.
As the supply continues to fall past VPFD, an internal
switching device forces VOUT to the external backup en-
ergy source. CEOUT is held high by the VOUT energy
source.
During power-up, VOUT is switched back to the 5V sup-
ply as VCC rises above the backup cell input voltage
sourcing VOUT. CEOUT is held inactive for time tCER af-
ter the power supply has reached VPFD, independent of
the CEIN input, to allow for processor stabilization.
During power-valid operation, the CEIN input is passed
through to the CEOUT output with a propagation delay of
less than 12ns.
Figure 2 shows the hardware hookup for the external
RAM, battery, and crystal.
A primary backup energy source input is provided on
the bq4845. The BC input accepts a 3V primary battery,
typically some type of lithium chemistry. Since the
bq4845 provides for reverse battery charging protection,
no diode or current limiting resistor is needed in series
with the cell. To prevent battery drain when there is no
valid data to retain, VOUT and CEOUT are internally iso-
lated from BC by the initial connection of a battery. Fol-
lowing the first application of VCC above VPFD, this iso-
lation is broken, and the backup cell provides power to
VOUT and CEOUT for the external SRAM.
The crystal should be located as close to X1 and X2 as
possible and meet the specifications in the Crystal
Specification Table. With the specified crystal, the
bq4845 RTC will be accurate to within one minute per
month at room temperature. In the absence of a crystal,
a 32.768 kHz waveform can be fed into X1 with X2
grounded.
Power-On Reset
The bq4845 provides a power-on reset, which pulls the
RST pin low on power-down and remains low on power-
up for tRST after VCC passes VPFD. With valid battery
voltage on BC, RST remains valid for VCC= VSS.
Figure 2. bq4845 Application Circuit
6
Aug. 1995