add beginnings of a uart-controlled prototyping app

apps/uart-display/Makefile

@@ -0,0 +1,10 @@
+TOP = ../..
+include $(TOP)/make.mk
+
+INCLUDES += \
+  -I./
+
+SRCS += \
+  ./app.c
+
+include $(TOP)/rules.mk

apps/uart-display/app.c

@@ -0,0 +1,104 @@
+#include <stdio.h>
+#include <string.h>
+#include <peripheral_clk_config.h>
+#include "watch.h"
+
+/*
+Beginnings of a program to take UART input and update the screen accordingly.
+Use alongside a prpgram that communicates with the watch over the UART pins.
+This CircuitPython script turns the LED red in response to pressing the LIGHT button,
+and turns it off when the MODE button is pressed:
+
+```
+import board
+import busio
+
+uart = busio.UART(board.TX, board.RX, baudrate=19200)
+
+while True:
+    uart.write(b"\x00")
+    data = uart.read(1)
+
+    if data is not None:
+        data_string = ''.join([chr(b) for b in data])
+        print(data_string, end="")
+        if data_string[0] == 'L':
+            uart.write(b"R")
+        elif data_string[0] == 'M':
+            uart.write(b"O")
+```
+*/
+
+
+char button_pressed = 0;
+
+static void cb_light_pressed(void) {
+    button_pressed = 'L';
+}
+
+static void cb_mode_pressed(void) {
+    button_pressed = 'M';
+}
+
+static void cb_alarm_pressed(void) {
+    button_pressed = 'A';
+}
+
+void app_init(void) {
+    watch_enable_leds();
+    watch_enable_buzzer();
+
+    watch_enable_external_interrupts();
+    watch_register_interrupt_callback(BTN_MODE, cb_mode_pressed, INTERRUPT_TRIGGER_RISING);
+    watch_register_interrupt_callback(BTN_LIGHT, cb_light_pressed, INTERRUPT_TRIGGER_RISING);
+    watch_register_interrupt_callback(BTN_ALARM, cb_alarm_pressed, INTERRUPT_TRIGGER_RISING);
+
+    watch_enable_display();
+
+    watch_enable_uart(A2, A1, 19200);
+}
+
+void app_wake_from_backup(void) {
+}
+
+void app_setup(void) {
+}
+
+void app_prepare_for_standby(void) {
+}
+
+void app_wake_from_standby(void) {
+}
+
+bool app_loop(void) {
+    char buf[3];
+
+    if (button_pressed) {
+        sprintf(buf, "%c", button_pressed);
+        printf("%s\n", buf);
+        watch_uart_puts(buf);
+        button_pressed = 0;
+    }
+    char char_received = watch_uart_getc();
+    if (char_received) {
+        switch (char_received) {
+            case 'R':
+                watch_set_led_red();
+                break;
+            case 'G':
+                watch_set_led_green();
+                break;
+            case 'Y':
+                watch_set_led_yellow();
+                break;
+            case 'O':
+                watch_set_led_off();
+                break;
+            case 'U':
+                // receive a display update?
+                break;
+        }
+    }
+
+    return false;
+}