Preliminary
CBC34813 EnerChip™ RTC
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
CBC34813ꢀ(AM0813)ꢀRegisterꢀDefinitionsꢀ(00ꢀtoꢀ0F)
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.
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DS-72-38 V.01
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