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Help me solve some issues with 68HC11A1 bootstrap

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Oct 9, 2007
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I am having a bit of a problem getting the development board I designed to fire up some 68HC11A1 chips I had. I am confident everything is ok but when MODA and MODB are pulled low and I reset the thing I can't seem to get the break character on the SCI which would indicate I have got it into bootstrap mode. I observed the following with a multimeter and Scope when I supply power


- MODB and MODA pins are both pulled low(bootstrap mode)
- Reset is High(means we have come out of reset)
- I get the correct waveform on the E pin (8mhtz clock / 4 ) pin indicating that I think that the crystal network is ok.
- After reset I monitor TX pin on SCI and it holds at 5 volts(bad that it doesn’t go low though)


- I monitor the TX pin on the Chips SCI with the scope and set it to trigger recording at 1 MHz(very conservative) on falling edge, then I fire up the power to 68Hc11 chip but it never triggers(I take this to mean that something is wrong as the bootloader code should make TXD go high on power up then quite quickly pull it low(falling edge) to indicate the transition of the break character so the user can then download code via SCI

- I monitor MODA on power up(triggered on falling edge) which then turns the PIN into LIR(Active Low)(Load Instruction resister) which should be pulled low every time an instruction is processed inside the chip( Should see a change on this as the bootloader code executes but doesn’t seem to happen)

THE BIGGEST thing I am wondering is if the A1 version has the bootload ROM at all and can it be invoked by the normal bootloader process of pulling moda and modb low at reset. All the doco I read says that the A1 version has the ROM disabled, does this mean the normal masked ROM area that would be created in the factory when the end product code is finished and it is etched in(normally a complete waste of space for a hobbyist like me) or does this mean that the Bootloader ROM that in normal mode is invisible and is only mapped in when special bootstrap mode is enable.



To enter bootstrap mode Tx and Rx must be connected together and pulled high with a 3k3 resistor.

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