Chip Select Circuit - Rabbit RabbitCore RCM2200 User Manual

C.2 Chip Select Circuit
The RCM2100 has provision for battery backup, which kicks in to keep VRAM from
dropping below 2 V.
When the RCM2200 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 Vcc 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 switch accomplishes this task for the CS signal line.
Figure C-3 shows a schematic of the chip select control switch.
VRAM
R28 100 kW
/CSRAM
Q4
/CS1
Q3
VRAM
SWITCH
/RESET _ OUT
Figure C-3. Chip Select Control Switch
In a powered-up condition, the CS control switch 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 as high as the battery voltage). Q3 and Q4 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 (R28). 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 Q3 and Q4 are of opposite polarity so that a rail-to-rail voltages can be passed.
When the /CS1 voltage is low, Q3 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.
User's Manual 43