VHDL implementation of TI Datamath and Sinclair Scientific vintage calculators
Both of these used a variation of the TMS0800 chip, with different program and instruction mask. All of that has been replicated. (Find more info here: https://hackaday.io/project/167457-tms0800-fpga-implementation-in-vhdl
This is the reverse engineering of the US patent #3934233 which can be found here: https://patents.google.com/patent/US3934233
The project has been inspired by the cool JScript / HTML simulation by Ken Shirriff: http://files.righto.com/calculator/TI_calculator_simulator.html
The implementation is not replicating 100% the internals of the original chip, as random MOS logic does not translate too well into FPGA RTL. Instead, a classic micro-coded approach was used, with a 256 * 52 control store. The upper part of this store (0x80 - 0xFF) maps directly into TMS0800 instructions (e.g. entry point for ZFA (0x58X) is at 0xD8), and the lower part is used to implement the instructions. Some microinstructions are different for TI and Sinclair, for those the "sinclair" pin is examined as a condition to microinstruction branch, leading to different implmentations.
Both program stores are present at the same time (512 * 12 bit words, only 320 are used), the instruction pointer drives both, but based on "sinclair" pin, the correct one is multiplexed into instruction decoding and masking logic. There are also 2 mask tables, one for TI one for Sinclair. The calculator program(s) can be downloaded and "compiled" to be consumable by VHDL build time running the AsmGenerator.exe utility.
The "TMS0800" is wrapped into additional logic to allow it to be used as a calculator - for that the I/O devices on Mercury base-board need to be adapted to the calculator core. This includes:
- debug tracer on VGA (therefore a simple text-based VGA controller and 4k of video character RAM + chargen ROM are present too)
- PmodKYPD keyboard on PMOD
- 7 seg LED driver
Some of these are mutually exclusive as they use single PMOD. Setting all switches to off and pressing button 3 starts calculator with:
- TI mode
- 12.5MHz CPU frequency
- debug disabled, single step off
- VGA tracing disabled
- 7 seg showing calculator input/results
Either tracer circuit allows observation of internal state after each instruction. This is also driven by microcode routine, locations 0x0A to 0x3F. Here is the startup sequence:
PC=000 I=58F A=9999999999 B=9999999999 C=9999999999 AF=0000000000 BF=1111111111 CF=0 PC=001 I=57F A=9999999999 B=9999999999 C=9999999999 AF=0000000000 BF=0000000000 CF=0 PC=002 I=70F A=0000000000 B=9999999999 C=9999999999 AF=0000000000 BF=0000000000 CF=0 PC=003 I=72F A=0000000000 B=9999999999 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=005 I=6DB A=0000000000 B=9999999999 C=0000000000 AF=0000000000 BF=0000000000 CF=1 PC=013 I=70F A=0000000000 B=9999999999 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=014 I=586 A=0000000000 B=9999999999 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=015 I=73F A=9999999999 B=0000000000 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=016 I=70B A=0000000199 B=0000000000 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=017 I=61B A=0000000299 B=0000000000 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=018 I=73C A=0000000099 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=019 I=63E A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=01A I=69D A=0000000099 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=1 PC=01E I=63E A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=01F I=598 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=020 I=229 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=021 I=550 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=022 I=560 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=023 I=221 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=024 I=550 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=025 I=560 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=026 I=221 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=027 I=66E A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=028 I=72E A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0 PC=029 I=578 A=0000000000 B=0000000200 C=0000000000 AF=0000000000 BF=0000000000 CF=0
at this point TMS0800 is executing the following instruction: X"02A" => X"527" --WAITNO F9 .0......... which means it is waiting in the loop for any key press.
Typical instructions takes at least 14 cycles (11 to go over digit plus 2 fork and next overhead), but the display and keyboard scan is driven at the fork microinstruction which happens only once per instruction execution, therefore from outside the basic timing relationship between execution and scan is preserved. This is visible in this sequence:
... '112 SYNC ', '113 BKO CLEAR ; Clear key pressed?', '114 BKO EQLKEY ; Equal key pressed?', '115 BKO PLSKEY ; Plus key pressed?', '116 BKO MINKEY ; Minus key pressed?', '117 BKO MLTKEY ; Mult key pressed?', '118 BKO DIVKEY ; Divide key pressed?', '119 BKO CEKEY ; CE key pressed?', '120 BKO DPTKEY ; Decimal point key pressed?', '121 BKO ZERKEY ; Zero key pressed?', ...
SYNC is done when the keyboard scan is about to start a new round. After that each instruction advances the scan to next key, so a single sense line controlling the BKO instruction can branch to the right routine.
Amateurish demo video here: https://www.youtube.com/watch?v=eGSghj2WLYs&t=49s
VGA debug display is seen in the video. The VGA controller is a simple 640 * 480 25MHz pixel clock one, generating 80 * 50 text display (this fits and utilizes 4k RAM well). There is also a built-in 8 * 8 character generator ROM. If debug is turned on, calculator branches into a trace routine (locations 0x0a - 0x3f) that sends values of all registers to the tracer circuit. This is essentially a "printer" to video RAM, so for example all printable ASCII codes after written to row, col (cursor pos) increment col, and row if col > 79. Scrolling is implemented too, when row > 49. Essentially same microcontrol unit is used like in TMS0800 but with much smaller microcode store.