Datasheet

STM690A/692A/703/704/802/805/817/818/819 Operation
Doc ID 10522 Rev 10 15/43
2.5 Chip-enable gating (STM818 only)
Internal gating of the chip-enable (E) signal prevents erroneous data from corrupting the
external CMOS RAM in the event of an undervoltage condition. The STM818 uses a series
transmission gate from E
to E
CON
(see Figure 11). During normal operation (reset not
asserted), the E
transmission gate is enabled and passes all E transitions. When reset is
asserted, this path becomes disabled, preventing erroneous data from corrupting the CMOS
RAM. The short propagation delay from E
to E
CON
enables the STM818 to be used with
most µPs. If E
is low when reset asserts, E
CON
remains low for typically 15 µs (or until E
goes high) to permit the current WRITE cycle to complete. Connect E
to V
SS
if unused.
2.6 Chip-enable input (STM818 only)
The chip-enable transmission gate is disabled and E is high impedance (disabled mode)
while reset is asserted. During a power-down sequence when V
CC
passes the reset
threshold, the chip-enable transmission gate disables and E
immediately becomes high
impedance if the voltage at E
is high. If E is low when reset asserts, the chip-enable
transmission gate will disable 15 µs after reset asserts (see Figure 12). This permits the
current WRITE cycle to complete during power-down.
Any time a reset is generated, the chip-enable transmission gate remains disabled and E
remains high impedance (regardless of E
activity) for the reset time-out period. When the
chip-enable transmission gate is enabled, the impedance of E
appears as a 40 Ω resistor in
series with the load at E
CON
. The propagation delay through the chip-enable transmission
gate depends on V
CC
, the source impedance of the drive connected to E, and the loading
on E
CON
. The chip-enable propagation delay is production tested from the 50% point on E to
the 50% point on E
CON
using a 50 Ω driver and a 50 pF load capacitance (see Figure 39).
For minimum propagation delay, minimize the capacitive load at E
CON
and use a low-output
impedance driver.
2.7 Chip-enable output (STM818 only)
When the chip-enable transmission gate is enabled, the impedance of E
CON
is equivalent to
a 40 Ω resistor in series with the source driving E
. In the disabled mode, the transmission
gate is off and an active pull-up connects E
CON
to V
OUT
(see Figure 11). This pull-up turns
off when the transmission gate is enabled.
Figure 11. Chip-enable gating
AI08802
V
RST
V
OUT
V
CC
COMPARE
E
CON
t
rec
Generator
E
CON
OUTPUT
CONTROL
RST
E