After flashing a new u-boot or after replugging the card my Stamp9261 or my Panel-Card doesn't boot anymore. What should I do?

After flashing a new u-boot or after replugging the card my Stamp9261 or my Panel-Card doesn't boot anymore. What should I do?

Description

The processor core of the AT91Sam9261 is operating at 1.8V. The peripherals like flash and SDRAM are operating at 3.3V. During power-up 1.8V are already stable and the processor core starts, but the peripherals aren't ready yet, so the system crashes. Actually, as the watchdog is enabled by default, the system will start after the watchdog timeout, which is 16 seconds, in a sane state.

Solution

This problem is solved by setting the reset register in the processor to user reset and insert a timeout before starting the core after the core voltage is stable. This is done before booting Linux, have a look at the setreset environment variable in u-boot. This register is battery backed-up, so it be will kept during power failures. There are three situations however, when this is not the case:

  • When the module was removed from the board
  • When the bootloader was newly flashed
  • When the battery was empty and replaced with a new one

When one of this situation occurs, you have to wait until the watchdog timeout or disconnect the battery temporarily, e.g. with a slip of paper between battery and connector.

If there is no battery the register doesn't need to be set as the settling time of the slowclock oscillator will guarantee a sane start of the board.

Caveat

If you own a revision of the Panel-Card below four, the manual reset is not needed and won't work. You find the revision number of your board on the LCD-Side of the Panel-Card, like 16.103.4 for revision four or 16.103.3 for revision three.

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