101-0506 Rabbit Semiconductor, 101-0506 Datasheet - Page 171

• Manufacturer Part Number: 101-0506
• Description: CARD CPU SR9160 128K SRAM SMRTST
• Manufacturer: Rabbit Semiconductor

Specifications of 101-0506

• Accessory Type: Smart Star CPU Card
• For Use With/related Products: SR9160
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

C.3 Chip Select Circuit
Figure C-5 shows a schematic of the chip select circuit for the RAM.
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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 kΩ
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
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