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Comment: the tower boards with the first rev of VF6 silicon will not hold the ZMK if using the battery.

Question asked by John Fielden on Nov 1, 2013
Latest reply on Nov 16, 2013 by John Fielden

As previously noted in this forum, the present rev of VF6 silicon pulls much more current on the Vbat pin than is specified in the data sheet.  The tower boards include a coin cell holder and a coin battery to allow experimenting with SNVS LP system in battery backed mode.  In particular, we are interested in maintaining the ZMK. If you try to use the coin cell with the current crop of tower boards from Freescale, you will not be able to maintain settings when cycling power.   The reason is that a current limiting series resistor for the battery is too large for multi-mA current pull. 


Our board appeared to be pulling 1.3mA from the battery when powered off.  This caused the Vbat to drop to 0.9V.  Decreasing R145 from 1.8K to 80 ohms, we saw the Vbat current increase to 5mA and the Vbat voltage rise to 2.6V.  At this current draw the battery voltage sagsl to just 3V.   The Vbat is enough to maintain the ZMK, but not for long.   The ZMK is maintained as expected at the higher Vbat.