Add pio onewire example (#334)

* Adds PIO onewire example

Co-authored-by: martin <admin@crossleys.biz>

pio/onewire/onewire.c

@@ -0,0 +1,77 @@
+/**
+ * Copyright (c) 2023 mjcross
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+**/
+
+#include <stdio.h>
+#include "pico/stdlib.h"
+#include "pico/binary_info.h"
+
+#include "onewire_library.h"    // onewire library functions
+#include "ow_rom.h"             // onewire ROM command codes
+#include "ds18b20.h"            // ds18b20 function codes
+
+// Demonstrates the PIO onewire driver by taking readings from a set of
+// ds18b20 1-wire temperature sensors.
+
+int main() {
+    stdio_init_all();
+
+    PIO pio = pio0;
+    uint gpio = 15;
+
+    OW ow;
+    uint offset;
+    // add the program to the PIO shared address space
+    if (pio_can_add_program (pio, &onewire_program)) {
+        offset = pio_add_program (pio, &onewire_program);
+
+        // claim a state machine and initialise a driver instance
+        if (ow_init (&ow, pio, offset, gpio)) {
+
+            // find and display 64-bit device addresses
+            int maxdevs = 10;
+            uint64_t romcode[maxdevs];
+            int num_devs = ow_romsearch (&ow, romcode, maxdevs, OW_SEARCH_ROM);
+
+            printf("Found %d devices\n", num_devs);      
+            for (int i = 0; i < num_devs; i += 1) {
+                printf("\t%d: 0x%llx\n", i, romcode[i]);
+            }
+            putchar ('\n');
+
+            while (num_devs > 0) {
+                // start temperature conversion in parallel on all devices
+                // (see ds18b20 datasheet)
+                ow_reset (&ow);
+                ow_send (&ow, OW_SKIP_ROM);
+                ow_send (&ow, DS18B20_CONVERT_T);
+
+                // wait for the conversions to finish
+                while (ow_read(&ow) == 0);
+
+                // read the result from each device
+                for (int i = 0; i < num_devs; i += 1) {
+                    ow_reset (&ow);
+                    ow_send (&ow, OW_MATCH_ROM);
+                    for (int b = 0; b < 64; b += 8) {
+                        ow_send (&ow, romcode[i] >> b);
+                    }
+                    ow_send (&ow, DS18B20_READ_SCRATCHPAD);
+                    int16_t temp = 0;
+                    temp = ow_read (&ow) | (ow_read (&ow) << 8);
+                    printf ("\t%d: %f", i, temp / 16.0);
+                }
+                putchar ('\n');
+            }
+            
+        } else {
+            puts ("could not initialise the driver");
+        }
+    } else {
+        puts ("could not add the program");
+    }
+
+    while(true);
+}