User`s manual

User’s Manual 83
VRAM is also available on pin 29 of header J2 to facilitate battery backup of the external
circuit. Note that the recommended maximum external current draw from VRAM is 100 µA,
and new battery-life calculations should be done to take external loading into account.
C.1.2 Power to VRAM Switch
The VRAM switch, shown in Figure C-3, allows a customer-supplied external battery to
provide power when the external power goes off. The switch provides an isolation
between Vcc and the battery when Vcc goes low. This prevents the Vcc line from draining
the battery.
Figure C-3. VRAM Switch
Transistor Q2 is needed to provide a very small voltage drop between Vcc and VRAM
(<100 mV, typically 10 mV) so that the processor lines powered by Vcc will not have a
significantly different voltage than VRAM.
When the RCM2100 is not resetting (pin 2 on U3 is high), the /RESET line will be high.
This turns on Q3, causing its collector to go low. This turns on Q2, allowing VRAM to
nearly equal Vcc.
When the RCM2100 is resetting, the /RESET line will go low. This turns off Q2 and Q3,
providing an isolation between Vcc and VRAM.
The battery-backup circuit keeps VRAM from dropping below 2 V.
C.1.3 Reset Generator
The RCM2100 uses a reset generator, U2, to reset the Rabbit 2000 microprocessor when
the voltage drops below the voltage necessary for reliable operation. The reset occurs
between 4.50 V and 4.75 V, typically 4.63 V. The RCM2100 has a reset output, pin 33 on
header J2. The reset generator has a reset input, pin 34 on header J2, that can be used to
force the RCM2100 to reset.
FDV302P
Q2
R15
R14
VRAM
VCC
Q3
MMBT3904
22 kW
/RESET
10 kW