Verilog UART to DBus (TI transfer bus, TI Link)

Use FPGA of choice to link a UART with a TI calculator.

Features

• Should work with all TI calculators featuring TI Link.
  • Tested with TI-89 (HW2) and TI Voyage 200 (HW2).
• Fast. Faster than official cables. Fastest. Calculator as bottleneck.
  • ~77Kbit/s from TI-89 (HW2 stock) with FPGA running dbus at 8MHz.
• Verilog 2005.
  • No dependency on vendor libraries.
  • Made for yosys+nextpnr open fpga flow.
  • Tested on iCE40 HX/LP/UP FPGAs.
• Custom dbus and uart implementations.
• Parametrized UART rate, DBus clock, buffer sizes.
• Hardware flow control.
• Ring FIFO buffer in both directions.
  • Buffer infers FPGA-specific dual port RAM.
• Timeout (2s) on dbus bit transfer:
  • Signal error (ground both lines for ~300µs) and dbus reset.
• MIT license. Refer to LICENSE file.

Tested FPGA boards

• BlackIce Mx
  • No UART from stm32 as it is hardcoded to 115200 and doesn't do RTS/CTS.
• iCEstick
• iCESugar
  • This board builtin UART through stm32 does not do RTS/CTS.
  • UART limited to 57600bps as workaround.
• TinyFPGA BX
• (...)

Recommendations

• TI-89, TI-92+ or Voyage 200 are fast enough to take advantage of this.
  • These calculators work reliably with dbus running higher than default, at 8+ MHz.
  • This is unnecessary. Performance benefits are negligible.
• Be careful with FPGA i/o pin tolerance.
  • TI dbus is maintained at 3.3v with m68k calcs.
  • TI dbus is at 5v (!) on some z80 calcs, including TI-86.
• Edit Makefile to uncomment or add fpga board.
• Edit board PCF to suit board board and preference.
• Preferably connect 2.5mm jack sleeve to GND rather than o_sleeve.
• Use pullup resistors (10kΩ suggested) on dbus tip/ring.
• Link68 pretty much just works:
  • https://github.com/jherning/link68
• If using tilp/libticables2:
  • No support for plain uart. Pretend it's a grey cable.
  • Patch grey cable for the uart device/rate/hwflow.
    • Like this (ttyUSB/115200/rtscts): https://gist.github.com/rvalles/f937889712d24ac6824f1358c936b3e2
  • If using an uart without hw flow control support with TILP:
    • Use this alternative patch (ttyACM/57600/noflow): https://gist.github.com/rvalles/0a7b076810470e8e4e2e0f1662eb70da
    • See caveats.
• If using TI official software:
  • Pretend it's a grey graphlink cable.
  • Use 9600bps, like a grey graphlink cable.

Caveats

• tilp/libticables2 likes to send files in large packets.
  • Some devboards will only accomodate a limited size ring buffer to calculator, which will fill faster than can be sent.
  • Flow control takes care of it, and thus it is not an issue.
  • If uart flow control is not possible, workaround is to set a slower uart speed in Makefile and in libticables2.
    • 57600bps is slow enough for m68k calculator models.

TODO

• Write some basic link software as an alternative to TILP.

Thanks

• Tim Singer/Romain Liévin for TI-73...V200 protocol guide.