Fast serial port / SD card adapter

http://myelin.nz/acorn/bbcserial

This is something I'm working on to provide a fast serial port with reliable flow control for a BBC or Electron. On the BBC, it attaches to the 1MHz bus, and on the Electron, it'll be a cartridge.

In both cases it consists of:

• a CPLD to communicate with the host machine's bus and share the SD card between the host machine and the Pro Micro.

• a Pro Micro (ATMEGA32U4) to provide a USB serial port, and in future act as a HostFS server.

The next version will replace the Pro Micro with an ATSAMD11 or ATSAMD21, and possibly add an ESP-WROOM-32 for wireless connectivity.

At present, it works well with a hacked-up version of J.G.Harston's HostFS, running against sweh's TubeHost code on an attached laptop. This provides quite a quick filing system that leaves PAGE=&E00 without requiring sideways RAM, as all the buffers are in the host machine's memory.

I haven't tested this yet, but it should also work with the Electron Plus 1 build of MMFS.

I'm still figuring out if I can distribute HostFS or TubeHost as part of this repository, but for the moment here's what you need to do:

• Download the HostFS:UPURS source from sweh's site (you want source.zip).

  • Unzip source.zip and edit hostfs.txt. Replace &FE08 (TxStatus and RxStatus) with &FCA1, and replace &FE09 (TxData and RxData) with &FCA0.

  • Create the OBJ folder and run 'make OBJ/HOSTFS'. This should build a HostFS ROM image in OBJ/HOSTFS. You'll need beebasm installed.

  • Load OBJ/HOSTFS into a spare sideways RAM bank, or program it into flash or EEPROM.

• Download TubeHost from sweh's site.

  • On macOS, run 'perl TubeHost -U /dev/tty.usb*'. Linux will be similar but with a different path for the serial port. On Windows/Cygwin you might be able to use 'perl TubeHost -U COMx'.

  • Hit BREAK on your Acorn machine and try listing the current directory, accessing files etc. Let me know how it goes!

Electron version

This is the first one I tried, using a modified elk_pi_tube_direct cartridge. I haven't made a proper PCB for it yet.

The CPLD design is in cpld/, and the MCU firmware is in serial_sd_mcu/.

Discussion on the Stardot forums.

BBC version

The PCB is in bbc_1mhz_bus_pcb/ and the CPLD design is in bbc_1mhz_bus_cpld/. It uses the same MCU firmware as the Electron version, from serial_sd_mcu/.

Discussion on the Stardot forums.

Bill of materials

• 1 x XC9572XL-xxVQG44 (44-pin 0.8mm TQFP. Speed rating doesn't matter. I've been using Digikey part number 122-1448-ND, but that's out of stock right now, so 122-1981-ND is the next cheapest. Just search for XC9572XL VQG44 and anything there should be good.)
• 1 x 2x5 header for CPLD programming (Digikey ED1543-ND), or you can just use bare pin headers.
• 1 x MCP1700T-3302E/MB regulator (Digikey MCP1700T3302EMBCT-ND)
• 1 x right angle 2x17 female connector for the 1MHz bus (Digikey S5570-ND)
• 1 x micro SD socket (digikey HR1941CT-ND)
• 3 x 0805 100nF capacitors
• 2 x 0505 1uF capacitors
• 1 x Pro Micro board (from eBay/Aliexpress -- make sure you get the one that looks like the pic below, with the micro USB socket; I've seen another kind that doesn't include USB)
• 1 x 3-pin 0.1" spacing header for power
• 2 x 12-pin 0.1" spacing header + 2 x 12-pin 0.1" spacing receptacle for the Pro Micro

Assembly notes

• Pin 1 of the CPLD is marked with a little bubble on the PCB; it's in the bottom left corner. There's a matching mark on the chip.
• Refer to the picture below for orientation for the JTAG header and Pro Micro.
• If you want to make the SD card easily accessible, solder on the 2x3 EXTSD header and run wires to an SD socket mounted somewhere else on your machine. If you want your SD card out of the way, solder on the micro SD socket (pictured below).
• You can either solder the Pro Micro right onto the board by soldering two 12-pin headers onto the board, then dropping the Pro Micro on top and soldering it on, or you can make it removable by soldering the headers onto the underside of the Pro Micro board, and soldering two 12-pin receptacles onto the board. I have it set up this way because I'm experimenting with different microcontrollers, but it's safe to solder the Pro Micro directly onto the board if you prefer.

Programming the Xilinx XC9572XL CPLD

Pre-built .jed and .svf files for the CPLD are checked in to the bbc_1mhz_serial_cpld/ folder, so you don't need to install the Xilinx tools to program the CPLD.

If you're using a simple_cpld_programmer board, you can program the CPLD like this:

cd bbc_1mhz_serial_cpld
make program

If you have an FTDI JTAG cable, you can program the CPLD like this:

cd bbc_1mhz_serial_cpld
make program-xc3sprog-ftdi

See ../notes/pld_programming_and_jtag.md for details on the JTAG pinout and on how to build a simple_cpld_programmer board.

Installed in a BBC B

Renders of the BBC PCB