CBC34813 EnerChip™ RTC

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DS-72-38 V.01

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Preliminary

CBC34813(AM0813)RegisterDefinitions(00to0F)

The following register bits must be set prior to any switchover, even from VDD to the EnerChip.

BREF bits: The default value out of reset is 0000. This needs to be reprogrammed to 1111.

IOBM bit: The default value out of reset is 1. This needs to be reprogrammed to 0.

Crystal Oscillator Selection

The AM0813 should work with any standard 32.768kHz tuning fork crystal with a load capacitance rating from

0 - 12pF and an ESR from 0 - 90kohms. Recommendations are as follows:

• Crystal load capacitance rating: 0 - 12pF

• Crystal ESR rating: 0 – 90kohms max

• No additional loading capacitors on the board

• Stray PCB capacitance on XO/XI: 2pF or less (less is better)

Typically, an oscillator allowance (OA) of 260-290kohms is generated. Increasing the loading capacitance on

the XI/XO pins will decrease the OA and using crystals with a higher ESR will reduce the OA margin. The crystal

will not affect the AM0813 RTC current because a fixed bias current to the crystal is used. No external load

capacitance is required because the frequency offset from the crystal is digitally calibrated out, to within +/-

2ppm. Mainstream crystals (3.2mm x 1.5mm) generally have a maximum ESR rating of 70kohms. The smaller

2.0mm x 1.2mm crystals generally have a maximum ESR of 90kohms. Some crystal vendors, such as Epson

or Micro Crystal, might have some of the smaller crystals with lower ESR. Below is a list of crystals from several

vendors that have been tested:

Abracon: ABS07-32.768KHZ-7-T, ABS06-32.768KHZ-9-T, ABS25.32.768KHZ-T

Epson: C-002RX, FC-135, FC-12D, FC-12M

Micro Crystal: CC7V-T1A, CM7V-T1A