Wednesday, December 10, 2008

Weird backlight control on the Archos V2/FM Jukebox Recorder

When turning off, the backlight on my Archos V2 Jukebox recorder sometimes turns off and sometimes brightens and then fades out. I investigated this a bit today. The backlight is controlled by the square wave output of the STMicroelectronics M41ST84W real time clock. Rockbox configures it for 32kHz and then enables and disables it as needed. Apparently disabling it means high impedance mode (high-z/3-state) instead of off. What happens to the backlight depends on the last state of the square wave output. If it was off, it stays off. If it was on, it stays on and slowly fades as the charge leaks away. This is brighter at first because it is steady on, not flashing at 32kHz as usual.

Setting the frequency bits to 0000 does the same thing, and changing them doesn't reset the counter. The only software workaround would be to shut off the backlight at the right moment, and it may be possible to wait for the right moment based on the 10ths of a second value, then decrease the frequency and shut off the square wave. If the place from which the charge slowly leaks away is accessible outside the chip (eg. if the output drives a external MOSFET), it would be possible to add a resistor. However, I'm not sure this is worth doing.

Edited on December 13th:
I found a simple workaround and submitted a patch. Apparently, the square wave output is controlled by its own counter which is independent of the clock, and the counter is reset when the output is disabled. So, the solution is to disable the square wave to reset the counter, re-enable it so the backlight is off, and then finally disable it so the backlight stays off. Setting the frequency to 1Hz before the re-enabling ensures the backlight won't turn on before the final disabling.

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