{"id":2851,"date":"2024-10-11T17:42:10","date_gmt":"2024-10-11T16:42:10","guid":{"rendered":"https:\/\/www.robertprice.co.uk\/robblog\/?p=2851"},"modified":"2024-10-11T17:42:11","modified_gmt":"2024-10-11T16:42:11","slug":"rc2024-part-7-reading-the-rotary-encoder-using-z80-assembly-language","status":"publish","type":"post","link":"https:\/\/www.robertprice.co.uk\/robblog\/rc2024-part-7-reading-the-rotary-encoder-using-z80-assembly-language\/","title":{"rendered":"RC2024 \u2013 Part 7 \u2013 Reading The Rotary Encoder Using Z80 Assembly Language"},"content":{"rendered":"\n

Previously as part of this year’s Retro Challenge, I learnt how to build and run Z80 assembly language programs on my RC2014 Classic 2 computer<\/a>.<\/p>\n\n\n\n

I now want to recreate the BASIC program that moves LEDs<\/a> using Z80 Assembly Language.<\/p>\n\n\n\n

I am using IO address $DE for my Rotary Encoder module. I also have the Digital I\/O module using IO address $03. I can define these as constants so I can easily change them if necessary.<\/p>\n\n\n\n

I need to store values for the input, the output, and the last value of the CLK pin. The input will just be the value from the IN<\/a> operation. The output value will be the byte we want to show on the LED output. I will set this to be %00000001 initially. When I turn the encoder, I want this to shift to either the left or right. If it reaches the edge, I want it to wrap. The Z80 operations RLCA<\/a> and RRCA<\/a> will do this for me.<\/p>\n\n\n\n

To check if a bit is high or low, I can use an AND<\/a> operation to mask out out other values. For example, to check if the SW1 is being pressed, I can do the following.<\/p>\n\n\n\n

    SW1         EQU %00000100\n    INPUT_PORT  EQU $DE\n\n    in a,(INPUT_PORT)\n    and SW1\n    cp SW1\n    jp z, end    ; jump to end if SW is high.<\/code><\/pre>\n\n\n\n
I can repeat this logic to check the values of CLK1 and DT1.<\/p>\n\n\n\n
This is the code I have come up with. <\/p>\n\n\n\n
    OUTPUT rotaryencoderledcontrol.z80\n\n; On the RC2014 Classic 2 running from BASIC the Z80\n; code runs from address $9000. \n    ORG $9000\n\n; The input and output ports to use.\nINPUT_PORT  EQU $DE\nOUTPUT_PORT EQU $03\n\n; The input bits from the rotary encoder.\nCLK1    EQU %00000001\nDT1     EQU %00000010\nSW1     EQU %00000100\n\n; show the inital led value\n    ld a,(output)\n    out (OUTPUT_PORT),a\n\nloop:\n; load the last clk value into register b\n    ld a,(lastclk)\n    ld b,a\n\n; read the input port and store in \"input\"\n    in a,(INPUT_PORT)\n    ld (input),a\n\n; now check if the switch on first rotary encode has been\n; pressed. If it has jump to end\n    and SW1\n    cp SW1\n    jp z, end\n\n; now see if clk1 matches the lastclk. If it does loop\n    ld a,(input)\n    and CLK1\n    ld (lastclk),a\n    cp b\n    jp z, loop\n\n; now work out what direction we are moving\ncheck_case1:\n    ld  a,(input)          \n    and CLK1\n    cp  CLK1             \n    jr  nz, check_case2\n\n    ld  a,(input)         \n    and DT1\n    cp  DT1\n    jr  z, left     ; if both CLK and DT are high then left\n\ncheck_case2:\n    ld  a, (input)\n    and CLK1\n    cp  0 \n    jr  nz, right\n\n    ld  a, (input)\n    and DT1\n    cp  0 \n    jr  nz, right  \n\n; we must be turning left so rotate the output to the left\n; and store it before going back to the start of the loop.\nleft:\n    ld a,(output)\n    rlca\n    out (OUTPUT_PORT),a\n    ld (output),a\n\n    jp loop\n\n; we must be turning right so rotate the output to the right\n; and store it before going back to the start of the loop.\nright:\n    ld a,(output)\n    rrca\n    out (OUTPUT_PORT),a\n    ld (output),a\n\n    jp loop\n\n; the switch has been pressed, so we clear the output\n; and exit.\nend:\n    ld a,0\n    out (OUTPUT_PORT),a\n\n    ret\n\ninput:\n    db  0\noutput:\n    db  %00000001\nlastclk:\n    db  0\n<\/code><\/pre>\n\n\n\n
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