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https://github.com/etchedpixels/mini11

A 68HC11 SBC for the 68HC11A CPUs
https://github.com/etchedpixels/mini11

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A 68HC11 SBC for the 68HC11A CPUs

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README 

        
# Mini11: A Small 68HC11A System



The Mini11 is a small board that is designed to provide a complete large

68HC11 environment with a fairly low chip count. It provides 16K of ROM and

512K of RAM as well as breaking out all the spare I/O including the serial

and SPI in useful ways.



To keep cost down it's designed to use the many old 68HC11A series parts

that can be found in junk boxes and old boards. This has a different pinout

to the later processor variants.



## Requirements



- 68HC11A series processor (A0/A1/A8). 68HC811A probably works too

- 512K SRAM

- 27C256 or 28C256

- 74HC(T)139

- 74HC(T)00

- 74HC(T)573

- DS1233 reset controller

- 8MHz crystal

- pinheaders, sockets, resistors etc



## Construction



As the 68HC11 can be serial bootstrapped it is possible to assemble the

board and fit all the discrete parts and only the 68HC11 into its socket.

Fit jumpers JP1 and JP2 and apply power. You should be able to talk to the

board with db11 or similar tools.



If you have a part with ROM then you should also be able to boot from the

ROM if you boot with JP1 and JP2 removed. If the ROM contains Buffalo as

is usual then you will also need to jumper PE0 to ground. A 68HC11A8 with

Buffalo should then produce a boot message and you'll be able to talk to

the monitor.



The boot mode can also be used to check out the board after the other parts

are fitted. You can boot in single chip mode and then use db11 to turn off the

single chip mode and then check that the memory is behaving. Port A controls

the memory banking. Bit 3 turns the ROM on and off, bits 4-6 are the memory

bank.



With all the parts fitted you can burn firmware of choice into the ROM and

with the jumpers removed the chip should boot the firmware. Remember to use

the ".burn" file with the provided firmware or the tools to flip bit 0 and 2

as the board has D0 and D2 switched on the ROM



The provided firmware simply intializes the SD interface, loads the

first sector and runs it.



### Note



You cannot run an unmodified Buffalo from the ROM. It uses Port A pins for

tracing and this conflicts with the memory banking.



## ROM options



For a 27C256 you can load different firmware into each 16K bank and select

using JP3. For a 28C256 burn the firmware into the upper 16K and jumper JP3

to VCC.



## Rescuing Recalcitrant 68HC11A Parts



If you have problems with the 68HC11A such as it only coming up in bootstrap

mode then there are a few things to check.



### Wrong Config Byte



If the CONFIG byte is set to map in the internal ROM and the ROM contains

garbage or is absent (eg on a 68HC11A0/A1 part) then you can boot it into

single chip mode and use db11 to change the config byte to 0x0C or similar.



### Protected EEROM



The internal EEROM has protection options and some devices wih custom

firmware will set them. These can still be used as they will erase the

EEPROM when the serial boot is triggered. This will put 0F into the CONFIG

register so that will also then need reprogramming.



### Did You Remember D0/D2 Are Swapped In Your ROM Image



No.. opps



## External Interfaces



### J2: Serial (TTL Level)



Simple three wire interface with no flow control. Usually run at 9600 baud.



### J3: SPI (5v)



SPI. Provides the expected SPI interfaces as well as two chip select lines,

reset and the external IRQ (pulled up). Intended for an SD card and

optionally additional devices with an SPI mux/demux board. Remember to use

an SD adapter with voltage shifting.



### J4: Power



5v and ground, needs to be properly regulated. The board has an onboard

reset generator to handle power coming up gracefully.



## J5: GPIO



Three input ports mapped to port A bits 0-2



### J6: Analogue



Port E of the 68HC11.



## Jumpers



### JP1: MODA



Fit the jumper to set MODA low



### JP2: MODB



Fit the jumper to set MODB low



### JP3: ROM Select



Select bank on 27C256, set to 5V with a 28C256



## Software



At this point FUZIX. Build the "mini11" target, write the generated SD card

image raw to an SD card and boot the mini11 with it inserted. The console

runs at 9600 baud.