add PIO seven segment display example

Author: Martin Crossley

pio/seven_segment/7_segment.c

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+#include <stdio.h>
+#include "pico/stdlib.h"
+#include "7_segment_lib.h"
+
+
+const PIO pio = pio0;
+const uint first_segment_pin = 8;   // gpio 15-8 = segments E,D,B,G,A,C,F,dp
+const uint first_digit_pin = 16;    // gpio 19-16 = common anodes 4,3,2,1
+
+//                      EDBGACF.       bit ordering depends on your display and wiring
+const uint32_t Pico = 0b10111010 << 24 |    // 'P'
+                      0b10000000 << 16 |    // 'i'
+                      0b11010000 <<  8 |    // 'c'
+                      0b11010100;           // 'o'
+
+int main() {
+    uint sm;
+    stdio_init_all();
+    
+    if (seven_segment_init (pio, &sm, first_segment_pin, first_digit_pin)) {
+        puts ("running");
+
+        // display scrolling 'Pico'
+        for (int shift = 24; shift >= 0; shift -= 8) {
+            pio_sm_put (pio, sm, Pico >> shift);
+            sleep_ms (1000);
+        }
+
+        // count to 9999
+        for (int i = 0; i < 10000; i += 1) {
+            sleep_ms (100);
+            pio_sm_put (pio, sm, int_to_seven_segment (i));
+        }
+    }
+
+    puts ("done");
+    while (true);
+}

pio/seven_segment/7_segment_wiring.fzz

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+add_executable(pio_seven_segment)
+
+target_sources(pio_seven_segment PRIVATE 7_segment.c)
+
+add_subdirectory_exclude_platforms(pio_7_segment_library)
+
+target_link_libraries(pio_seven_segment PRIVATE
+	pico_stdlib 
+	hardware_pio 
+	pio_7_segment_library
+	)
+
+pico_add_extra_outputs(pio_seven_segment)
+
+# add url via pico_set_program_url
+example_auto_set_url(pio_seven_segment)

pio/seven_segment/7_segment_wiring_diagram.png

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+= Driving a 7 segment LED display
+This example demonstrates how a PIO state machine can be used to control a four digit multiplexed 7-segment display with the Raspberry Pi Pico (RP2040).
+
+Multiplexed displays work by rapidly displaying each digit in turn and the PIO provides an excellent way to relieve the CPU of this time-consuming task.
+
+The total current required is typically more than the GPIO pins can supply so you're likely to need an external drive circuit like the one below.
+
+The PIO code uses four `side-set` pins to control the digit multiplex lines and displays the segment patterns received on the FIFO. It uses a non-blocking PULL to keep showing the same segments until the CPU sends new data.
+
+The provided example uses spells out the word **Pico** and then counts from 0 to 9999.
+
+== Wiring information
+Connect the display to your board using a circuit like the one below, making any changes for your display and transistors.
+
+Connect the circuit to an external 5V supply or power it via USB (in which case reconnect the _+5V_ rail to _VBUS_ instead of _VSYS_).
+
+TIP: this circuit is for a _common anode_ display that takes about 15mA per segment.
+
+[[pio_7_segment_wiring-diagram]]
+[pdfwidth=75%]
+.Wiring diagram
+image::7_segment_wiring_diagram.png[]