101-0402 Rabbit Semiconductor, 101-0402 Datasheet - Page 68

• Manufacturer Part Number: 101-0402
• Description: KIT DEV TCP/IP 2000 INTL
• Manufacturer: Rabbit Semiconductor
• Series: RabbitCore 2000r
• Type: MPU Moduler

Specifications of 101-0402

• Contents: RabbitCore Module, Dev. Board, Cable and Dynamic C® CD-Rom
• For Use With/related Products: RCM2000, RCM2010, RCM2020
• Lead Free Status / RoHS Status: Vendor undefined / Vendor undefined

B.2.2 Power to VRAM Switch
The VRAM switch, 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.
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Figure B-4. VRAM Switch
Transistor Q3 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 TCP/IP Development Board is not resetting (pin 2 on U4 is high), the /RES line
will be high. This turns on Q4, causing its collector to go low. This turns on Q3, allowing
VRAM to nearly equal Vcc.
When the TCP/IP Development Board is resetting, the /RES line will go low. This turns
off Q3 and Q4, providing an isolation between Vcc and VRAM.
The battery backup circuit keeps VRAM from dropping below 2 V.
B.2.3 Reset Generator
The TCP/IP Development Board 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.
TCP/IP Development Kit
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