Rabbit eDisplay, User Manual

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eDisplay (OP7200)
B.2.4 Power to VRAM Switch
The VRAM switch on the OP7200’s RabbitCore module, shown in Figure B-4, allows the
battery backup 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 B-4. 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 OP7200 is not in reset, the /RES_OUT line will be high. This turns on Q2, caus-
ing its collector to go low. This turns on Q5, allowing VRAM to nearly equal Vcc.
When the OP7200 is in reset, the /RES_OUT line will go low. This turns off Q2 and Q5,
providing an isolation between Vcc and VRAM.
B.2.5 Reset Generator
The OP7200’s RabbitCore module uses a reset generator on the module, U1, to reset the
Rabbit 2000 microprocessor when the voltage drops below the voltage necessary for reli-
able 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 k W
0 W
/RES_OUT
10 k W