RabbitCore RCM3100 User's Manual

Appendix D. Power Supply

Appendix D provides information on the current requirements of the RCM3100, and includes some background on the chip select circuit used in power management.

D.1 Power Supplies

The RCM3100 requires a regulated 3.3 V ± 0.15 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 RCM3100 board through header J2.

An RCM3100 with no loading at the outputs operating at 29.4 MHz typically draws 75 mA. The RCM3100 will consume an additional 10 mA when the programming cable is used to connect the programming header, J3, to a PC.

D.1.1 Battery-Backup Circuits

The RCM3100 does not have a battery, but there is provision for a customer-supplied battery to back up SRAM and keep the internal Rabbit 3000 real-time clock running.

Header J2, shown in Figure D-1, allows access to the external battery. This header makes it possible to connect an external 3 V power supply. This allows the SRAM and the internal Rabbit 3000 real-time clock to retain data with the RCM3100 powered down.

Figure D-1. External Battery Connections
at Header J5

A lithium battery with a nominal voltage of 3 V and a minimum capacity of 165 mA·h is recommended. A lithium battery is strongly recommended because of its nearly constant nominal voltage over most of its life.

The drain on the battery by the RCM3100 is typically 7.1 µA when no other power is supplied. If a 165 mA·h battery is used, the battery can last almost 3 years:

The actual life in your application will depend on the current drawn by components not on the RCM3100 and the storage capacity of the battery. The RCM3100 does not drain the battery while it is powered up normally.

Cycle the main power off/on on the RCM3100 after you install a backup battery for the first time, and whenever you replace the battery. This step will minimize the current drawn by the real-time clock oscillator circuit from the backup battery should the RCM3100 experience a loss of main power.

D.1.2 Reset Generator

The RCM3100 uses a reset generator to reset the Rabbit 3000 microprocessor when the voltage drops below the voltage necessary for reliable operation. The reset occurs between 2.55 V and 2.70 V, typically 2.63 V. The RCM3100 has a reset output, pin 1 on header J2.