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C.3 Chip Select Circuit
VRAM
R31
/CSRAM
Q4
/CS1
Q5
VRAM
R37
10 k W
/RES
Figure C-5. Chip Select Circuit
The current drain on the battery in a battery-backed circuit must be kept to a minimum.
When the CPU Card is not powered, the battery keeps the SRAM memory contents and
the real-time clock (RTC) going. The SRAM has a powerdown mode that greatly reduces
power consumption. This powerdown mode is activated by raising the chip select (CS)
signal line. Normally the SRAM requires +5 V to operate. However, only 2 V is required
for data retention in powerdown mode. Thus, when power is removed from the circuit, the
battery voltage needs to be provided to both the SRAM power pin and to the CS signal
line. The CS control circuit accomplishes this task for the CS signal line.
In a powered-up condition, the CS control circuit must allow the processor ’s chip select
signal /CS1 to control the SRAM’s CS signal /CSRAM. So, with power applied, /CSRAM
must be the same signal as /CS1, and with power removed, /CSRAM must be held high
(but only needs to be battery voltage high). Q4 and Q5 are MOSFET transistors with
opposing polarity. They are both turned on when power is applied to the circuit. They
allow the CS signal to pass from the processor to the SRAM so that the processor can peri-
odically access the SRAM. When power is removed from the circuit, the transistors will
turn off and isolate /CSRAM from the processor. The isolated /CSRAM line has a 100 k ?
pullup resistor to VRAM (R31). This pullup resistor keeps /CSRAM at the VRAM voltage
level (which under no power condition is the backup battery’s regulated voltage at a little
more than 2 V).
Transistors Q4 and Q5 are of opposite polarity so that a rail-to-rail voltage can be passed.
When the /CS1 voltage is low, Q5 will conduct. When the /CS1 voltage is high, Q4 will
conduct. It takes time for the transistors to turn on, creating a propagation delay. This
delay is typically very small, about 10 ns to 15 ns.
The signal that turns the transistors on is a high on the processor ’s reset line, /RES. When
the CPU Card is not in reset, the reset line will be high, turning on n-channel Q5. When a
reset occurs, the /RES line will go low.
User ’s Manual
171
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