Add PIO 7-segment display example

[mjcross]

Add an example showing how to use the PIO to control a multiplexed 7-segment display.
@lurch this re-creates #375 as discussed, due to loss of the original repo

[lurch]

@mjcross I've not tested this, as I don't have a bagful of transistors handy, but does this work?

// Simple example of how to convert an integer between -999 and 9999
// into a 32-bit word representing up to four 7-segment digits.
//
uint32_t int_to_seven_segment (int num) {
    uint32_t word = 0;
    if (num < -999 || num > 9999) {
        // number out of range, display 'E' symbol
        //       EDBGACF.
        word = 0b11011010
    } else { 
        if (num == 0) {
            word = segments[0];
        } else {
            bool negative = num < 0;
            if (negative) {
                num *= -1;
            }
            int bitshift;
            for (bitshift = 0; bitshift < 32 && num > 0; bitshift += 8) {
                word |= segments[num % 10] << bitshift;
                num /= 10;
            }
            if (negative) {
                bitshift += 8;
                // display '-' symbol
                //        EDBGACF.
                word |= 0b00010000 << bitshift;
            }
        }
    }
    return word;
}

[mjcross]

@lurch yes: tested a couple of years ago when I wrote it, with tiny 7-segment displays that didn't need driver transistors ;-) The project I wrote it for needed up using an OLED display instead so it never got used.
The only thing I changed was the ordering of the bit-patterns in the lookup tables, just to make the wiring look simpler in the circuit diagram.
I'll do a quick check on that function later today, just to be sure though.

[mjcross]

TBH if there's any bit of the project likely to cause issues it's probably the CMakeLists.txt files: I see at some point several of the example builds were adjusted to use add_subdirectory_exclude_platforms() so I've copied that, but I must admit I have no idea what it does.

[lurch]

yes: tested a couple of years ago when I wrote it

Ahhh, sorry for the miscommunication - I wasn't asking "Does your function work?", I was asking "Does my suggested change to your function, to add support for displaying negative numbers, work?" 😂

to use add_subdirectory_exclude_platforms() so I've copied that, but I must admit I have no idea what it does.

It's so that if an example uses a hardware-feature that a particular board / chip / platform doesn't support, we don't try building that example for that chip. See e.g. https://github.com/raspberrypi/pico-examples/blob/master/rtc/CMakeLists.txt where we prevent the RTC examples from building for the RP2350 (as it doesn't have an RTC) or https://github.com/raspberrypi/pico-examples/blob/master/sha/CMakeLists.txt where we prevent the SHA examples from building for the RP2040 (as it doesn't have SHA256 hardware).

P.S. I've just realised that you also need to edit the top-level README.md in your PR, to add a link to your new example to https://github.com/raspberrypi/pico-examples?tab=readme-ov-file#pio 🙂

[mjcross]

Ahh - sorry! Didn't read carefully enough. That code looks nice - I'll check it this afternoon: tks!

[mjcross]

I like it! Turns out you don't need the bitshift += 8; otherwise the minus sign falls off the end LoL

[mjcross]

You can simulate it with this slightly hacky code...

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

// segment patterns for digits 0-9
//
// By convention the segments are labelled as follows:
//
//  AAAA
// F    B
// F    B
//  GGGG
// E    C
// E    C
//  DDDD  .
//
// the bit ordering should match the way you connect up the GPIOs. Here it is
//    EDBGACF.
static const uint8_t segments[] = {
    0b11101110,     // 0
    0b00100100,     // 1
    0b11111000,     // 2
    0b01111100,     // 3
    0b00110110,     // 4
    0b01011110,     // 5
    0b11011110,     // 6
    0b00101100,     // 7
    0b11111110,     // 8
    0b01111110      // 9
};

// for display
static char rows[7][50];

// Simple example of how to convert an integer between -999 and 9999
// into a 32-bit word representing up to four 7-segment digits.
//
uint32_t int_to_seven_segment (int num) {
    uint32_t word = 0;
    if (num < -999 || num > 9999) {
        // number out of range, display 'E' symbol
        //       EDBGACF.
        word = 0b11011010;
    } else { 
        if (num == 0) {
            word = segments[0];
        } else {
            bool negative = num < 0;
            if (negative) {
                num *= -1;
            }
            int bitshift;
            for (bitshift = 0; bitshift < 32 && num > 0; bitshift += 8) {
                word |= segments[num % 10] << bitshift;
                num /= 10;
            }
            if (negative) {
                //bitshift += 8;
                // display '-' symbol
                //        EDBGACF.
                word |= 0b00010000 << bitshift;
            }
        }
    }
    return word;
}



void append_digit(uint8_t word) {
    static char temp_row[7][10];

    char e = word & 0b10000000 ? '|': ' ';
    char d = word & 0b01000000 ? '-': ' ';
    char b = word & 0b00100000 ? '|': ' ';
    char g = word & 0b00010000 ? '-': ' ';
    char a = word & 0b00001000 ? '-': ' ';
    char c = word & 0b00000100 ? '|': ' ';
    char f = word & 0b00000010 ? '|': ' ';

    snprintf(temp_row[0], sizeof(temp_row[0]), " %c%c%c%c   ", a, a, a, a);
    snprintf(temp_row[1], sizeof(temp_row[1]), "%c    %c  ", f, b);
    snprintf(temp_row[2], sizeof(temp_row[2]), "%c    %c  ", f, b);
    snprintf(temp_row[3], sizeof(temp_row[3]), " %c%c%c%c   ", g, g, g, g);
    snprintf(temp_row[4], sizeof(temp_row[4]), "%c    %c  ", e, c);
    snprintf(temp_row[5], sizeof(temp_row[5]), "%c    %c  ", e, c);
    snprintf(temp_row[6], sizeof(temp_row[6]), " %c%c%c%c   ", d, d, d, d);

    for (int i=0; i<7; i+=1) {
        strlcat(rows[i], temp_row[i], sizeof(rows[i]) - 1);
    }
}

void display_all(uint32_t word) {
    for (int i=0; i<7; i+=1) {
        rows[i][0] = '\0';
    }

    append_digit(word >> (3 * 8));
    append_digit(word >> (2 * 8));
    append_digit(word >> (1 * 8));
    append_digit(word);

    for (int i=0; i<7; i+=1) {
        puts(rows[i]);
    }
}


int main(int argc, char **argv) {

    display_all(int_to_seven_segment(-999));
    return (0);
}

[mjcross]

The output is actually rather cute

                 ----    ----   
             |       |       |  
             |       |       |  
 ----            ----    ----   
             |  |            |  
             |  |            |  
                 ----    ----  

[mjcross]

Hope the complex-looking circuit doesn't put people off: it's a really good use-case for the PIO.
You're right that using a driver chip would have been a better idea than discrete transistors...