BQ4013 Datasheet, PDF (2/14 Pages) Texas Instruments – 128Kx8 Nonvolatile SRAM

bq4013/Y
Functional Description
When power is valid, the bq4013/Y operates as a stan-
dard CMOS SRAM. During power-down and power-up
cycles, the bq4013/Y acts as a nonvolatile memory, auto-
matically protecting and preserving the memory con-
tents.
Power-down/power-up control circuitry constantly moni-
tors the VCC supply for a power-fail-detect threshold
VPFD. The bq4013 monitors for VPFD = 4.62V typical for
use in systems with 5% supply tolerance. The bq4013Y
monitors for VPFD = 4.37V typical for use in systems
with 10% supply tolerance.
When VCC falls below the VPFD threshold, the SRAM au-
tomatically write-protects the data. All outputs become
high impedance, and all inputs are treated as “don’t
care.” If a valid access is in process at the time of
power-fail detection, the memory cycle continues to com-
pletion. If the memory cycle fails to terminate within
time tWPT, write-protection takes place.
As VCC falls past VPFD and approaches 3V, the control
circuitry switches to the internal lithium backup supply,
which provides data retention until valid VCC is applied.
When VCC returns to a level above the internal backup
cell voltage, the supply is switched back to VCC. After
VCC ramps above the VPFD threshold, write-protection
continues for a time tCER (120ms maximum) to allow for
processor stabilization. Normal memory operation may
resume after this time.
The internal coin cell used by the bq4013/Y has an ex-
tremely long shelf life and provides data retention for
more than 10 years in the absence of system power.
As shipped from Unitrode, the integral lithium cell of
the MA-type module is electrically isolated from the
memory. (Self-discharge in this condition is approxi-
mately 0.5% per year.) Following the first application of
VCC, this isolation is broken, and the lithium backup cell
provides data retention on subsequent power-downs.
Block Diagram
OE
128K x 8
SRAM
WE _Block
A0–A16
DQ0–DQ7
Power
CECON
CE _{Power-Fail VCC}
Control
Lithium
Cell
BD-42
2