User`s manual

User’s Manual Power Supply B–1
Power Supply B
Appendix B provides information on the current requirements of the RabbitCore
RCM2100, and some background on the chip select circuit used in power mange-
ment.
B.1 Power Supplies
The RabbitCore RCM2100 requires a regulated 5 V
± 0.25 V DC power source. The RabbitCore design
presumes that the voltage regulator is on the user
board, and that the power is made available to the
RabbitCore board through headers J1 and J2.
A RabbitCore RCM2100 with no loading at the out-
puts operating at 22.1 MHz typically draws 140 mA .
The RabbitCore RCM2100 will consume 13 mA to
15 mA of additional current when the programming
cable is used to connect J5 to a PC.
B.2 Chip Select Circuit
As explained in Section 2.4.2, the RabbitCore
RCM2100 has provision for battery backup, which
kicks in to keep VRAM from dropping below 2 V.
The current drain on the battery in a battery-backed
circuit must be kept to a minimum. When the Rab-
bitCore RCM2100 is not powered, the battery keeps
the SRAM memory contents and the real-time clock
(RTC) going. The SRAM has a powerdown mode
that greatly reduces power consumption. This pow-
erdown mode is activated by raising the chip select
(CS) signal line. Normally the SRAM requires Vcc
to operate. However, only 2 V is required for data
retention in powerdown mode. Thus, when power is
removed from the circuit, the battery voltage needs
to be provided to both the SRAM power pin and to
the CS signal line. The CS control circuit accom-
plishes this task for the CS signal line.