68HC11 RC2014 CPU Board

[Alan Cox]

So I'm back from my detour into Skyrim. Alduin now looks like a hedgehog and it's back to PCB design...

This is the first crack at a 68HC11 CPU card and is loosely based on the 6303 CPU card but a lot harder to route. I/O is mapped into the $FExx range as with the other MMIO based cards I've done. Likewise it provides the RW and E signals on user pins so that 65xx and 68xx peripherals can be used directly.

A16-A19 are made available for the extended bus. The idea being that you can run from internal RAM and switch banks without any extra logic if you wanted. Equally it could run with the standard memory cards.

I need to work out the best clock frequency to use for the internal serial to be sane and a few other things (up to 8MHz although the CPU actually runs at clk/4). The HC11 is 6800 compatible including the ability to move internal I/O and memory so should be able to run a variety of things including Flex.

Anyone see any screwups in the design ?

Alan

[David Richards]

Hi Alan, iirc you may want a jumper on one of the mode pins to enable the serial bootstrap feature. Hth David.

[Bill Shen]

Alan,
It is close enough, nothing that can't be fix with a x-Acto knife and a bit of wires.  I think I have a few 68HC11 in PLCC package.  I'll go hunt around in my garage--probably come back with 3 bottles of wine or the Chinese coin collection I still can't find!
  Bill

[Nick Brok]

Hi Alan,

I suggest to look for the 68hc11 programming reference guide. The pre-scaler of the baudrate generator is so programmable that at 8MHz clock you can get 9600 baud max. It's called the baud register. On the 68hc11F1 the address of the BAUD register is x02fh. So I suggest to use 8 MHz as clock frequency. The schematic it self looks good to me.

The 68HC11 fetch at location 0FFFEh and 0FFFFh the start address of the ROM, so it's better to have (E)PROM at 0E000h-0FFFFh. (8K)

I suggest to map the I/O under 0E000H.

greetings,

Nick

Op maandag 3 februari 2020 23:59:14 UTC+1 schreef Alan Cox:

[Alan Cox]

On Tuesday, 4 February 2020 05:22:14 UTC, Nick Brok wrote:

Hi Alan,

I suggest to look for the 68hc11 programming reference guide. The pre-scaler of the baudrate generator is so programmable that at 8MHz clock you can get 9600 baud max. It's called the baud register. On the 68hc11F1 the address of the BAUD register is x02fh. So I suggest to use 8 MHz as clock frequency. The schematic it self looks good to me.

Thanks. I've found one schematic error so far - A19 will float at power up because the 68HC11E initializes that port as input so I think I need a 4K7 pulldown on it so that the CPU powers up fetching from ROM with a flat memory card.

For the baud I was hoping for better than 9600. Having done the maths with the chart it looks like using the standard RC2014 clock and the /1 divider gives me

115200, 57600 and then /3 divider gives me 38400, 19200, 9600, 4800, 2400, 1200, 600, 300, etc so covers the full normal baud rate set.

The 68HC11 fetch at location 0FFFEh and 0FFFFh the start address of the ROM, so it's better to have (E)PROM at 0E000h-0FFFFh. (8K)

I suggest to map the I/O under 0E000H.

This is a really good point. It would be very hard to work with the external peripherals using Buffalo as it will be mapped over the upper 12K. On the other hand it's much more convenient having it at $FExx for Fuzix and for compatibility. Let me see if I can fit a jumper in somehow.

Alan