{"id":2859,"date":"2024-10-12T12:27:01","date_gmt":"2024-10-12T11:27:01","guid":{"rendered":"https:\/\/www.robertprice.co.uk\/robblog\/?p=2859"},"modified":"2024-10-12T12:27:01","modified_gmt":"2024-10-12T11:27:01","slug":"rc2024-part-8-refining-the-rotary-encoder-z80-assembly-language-program","status":"publish","type":"post","link":"https:\/\/www.robertprice.co.uk\/robblog\/rc2024-part-8-refining-the-rotary-encoder-z80-assembly-language-program\/","title":{"rendered":"RC2024 \u2013 Part 8 \u2013 Refining The Rotary Encoder Z80 Assembly Language Program"},"content":{"rendered":"\n

In my last Retro Challenge post I was able to read the rotary encoder from Z80 assembly language on my RC2014 Classic 2 computer<\/a>. However, the code was too sensitive. This was because I was looking for all changes to the CLK1 signal, instead of just the transition from low to high.<\/p>\n\n\n\n

This turned out to be simpler than I though, and I was able to remove a large(ish) block of code. The old code I had a check_case1 and check_case2 that can be removed and replaced with the following.<\/p>\n\n\n\n

; now work out what direction we are moving.\n; if CLK1 is 1 then we can can check DT1 to get the\n; direction of rotation. If it's 0, we need to go \n; back to the start of the loop.\n    ld  a,(input)          \n    and CLK1\n    cp  CLK1 \n    jr  nz, loop            \n\n; this is where we check DT1. If 1 we are turning left.\n    ld  a, (input)\n    and DT1\n    cp  0 \n    jr  nz, left<\/code><\/pre>\n\n\n\n
In the first part I’m loading the input data from memory. I then mask off everything but the CLK1 bit and see if it is high. If it’s not, I go back to the start of the program.<\/p>\n\n\n\n
In the second part I do the same for the DT1 bit, but check if it’s low. If high, I jump to the turning left code. If it is low, I carry on the to the turning right code, which directly follows.<\/p>\n\n\n\n
The completed code looks like this.<\/p>\n\n\n\n
    OUTPUT rotaryencoderledcontrol2.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    jr  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    jr  z, loop\n\n; now work out what direction we are moving.\n; if CLK1 is 1 then we can can check DT1 to get the\n; direction of rotation. If it's 0, we need to go \n; back to the start of the loop.\n    ld  a,(input)          \n    and CLK1\n    cp  CLK1 \n    jr  nz, loop            \n\n; this is where we check DT1. If 1 we are turning left.\n    ld  a, (input)\n    and DT1\n    cp  0 \n    jr  nz, left\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    jr  loop\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; 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<\/code><\/pre>\n\n\n\n
I also swapped the JP instructions to JR instructions because the code is small. This saves a few bytes.<\/p>\n\n\n\n
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