Simple 68HC11 board schematics
The purpose of this design is to have a board which allows you to power-up the MCU and interface with it via RS-232. I wanted to have something as minimal as possible, but still extensible enough to expand with external components.
The design is pretty crude, with only clock generator, primitive reset circuit an RS232 transceiver and IO connectors for almost everything. One gimmick in this circuit is the split of port E into two 4-pin connectors. If you look into the pin layout of the 52-pin PLCC package the E port pin layout is anything but straight - hence the two connectors. The B and C ports could be used to connect external memory.
If you happen to have a different 68HC11 MCU, you can probably sill use this circuit. Just add or remove external connectors depending on the amount of IO ports your MCU has.
You can use JP1 to select the boot mode. For my projects I use "bootstrap" mode, which allows for a lot of external control.
This schematics has one flaw: the reset circuit. I used simple RC combination (R5, C3). The 68HC11 spec states you should not do so, but use external MCU control chip, which monitors the voltage and draws the reset signal as necessary. Well for one I don not have such a chip, second the RC works fine for me, but your mileage may vary.
One interesting idea for the reset signal can be found here. I have not tried this, but it looks handy that the reset can be performed via the serial port.
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