76 Smartcat (BL2100)
B.2.3 Power to VRAM Switch
The VRAM switch on the BL2100 module, shown in Figure B-3, allows the battery
backup to provide power when the external power goes off. The switch provides an isola-
tion between Vcc and the battery when Vcc goes low. This prevents the Vcc line from
draining the battery.
Figure B-3. VRAM Switch
Field-effect transistor Q5 is needed to provide a very small voltage drop between Vcc and
VRAM (<100 mV, typically 10 mV) so that the board components powered by Vcc will
not have a significantly different voltage than VRAM.
When the BL2100 is not in reset, the /RESET line will be high. This turns on Q2, causing
its collector to go low. This turns on Q5, allowing VRAM to nearly equal Vcc.
When the BL2100 is in reset, the
/RESET line will go low. This turns off Q2 and Q5, pro-
viding an isolation between Vcc and VRAM.
B.2.4 Reset Generator
The BL2100 module uses a reset generator on the module, U1, 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.
FDV302P
Q5
R33
R17
R30
VRAM
VCC
Q2
MMBT3904
22 kW
0 W
/RESET
10 kW
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