Datasheet

Operation STM690, STM704, STM795, STM802, STM804, STM805, STM806
16/42 Doc ID 10519 Rev 9
2.5 Chip enable gating (STM795 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 STM795 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 E
propagation delay from E to E
CON
enables the STM795 to be used with
most µPs. If E
is low when reset asserts, E
CON
remains low for typically 10 µs to permit the
current write cycle to complete.
2.6 Chip enable input (STM795 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 10 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 first half of the reset time-out
period (t
rec
/2). 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 35). For minimum propagation delay, minimize the capacitive
load at E
CON
and use a low-output impedance driver.
2.7 Chip enable output (STM795 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