„Bad“ Expansion Header Pins on the Beaglebone

While making my first „cap“ for the Beaglebone (working on a YWasp cap), I learned the hard way that not all pins on the expansion header of the bone are „good“ pins. After putting the cap (which I worked stand alone fine) on the bone, it refused to boot! When taking the MSP430G2553 out of its socket (while leaving the cap installed), the bone booted again. Then when placing the MSP430 back in the socket with the bone already running and booted (yes, yes I known …), the cap also worked.

So I put the cap and the bone away, until yesterday a colleague of mine, who knows the Beaglebone schematics very well, came up with the point, that there are some „bad“ pins on the expansion header which are more ore less unusable. This is because the pins from the CPU which determine the boot-sequence of the ROM-code boot-loader are connected to some of the expansion header pins! Thus, if you fiddle around with this pins, it is very likely that you change the boot-sequence to something useless (e.g. tell it to boot from a serial line).

Matching this pins with the ones my cap used, it turned out, that I was about to use four of the 16 boot configuration pins. After re-wiring those pins, the bone booted fine again, and the cap worked also. The „bad“ pins are shown in the schematic below.

Beaglebone Boot Configuration

Beaglebone Boot Configuration

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