User`s manual

46 RabbitCore 2000
can periodically access the SRAM. When power is removed from the circuit, the transis-
tors 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 Q13 and Q14 are of opposite polarity so that a rail-to-rail voltages can be
passed. When the /CS1 voltage is low, Q13 will conduct. When the /CS1 voltage is high,
Q14 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 15ns.
The signal that turns the transistors on is a high on the processor’s reset line, /RES_OUT.
When the RabbitCore 2000 is not in reset, the reset line will be high, turning on n-channel
Q13 and Q15. Q15 is a simple inverter needed to turn on Q14, a p-channel MOSFET.
When a reset occurs, the /RES_OUT line will go low. This will cause C14 to discharge
through R31 and R33. This small delay (about 160 µs) ensures that there is adequate time
for the processor to write any last byte pending to the SRAM before the processor puts
itself into a reset state. When coming out of reset, CS will be enabled very quickly
because D13 conducts to charge capacitor C14.