Fix #115: wake from light sleep if a character arrives on the serial port

Note - we do this not by using the uart wake feature, but by the lower
power GPIO edge feature.  Recommend sending "Z" 0x5A - because that has
many edges.  Send the character 4 times to make sure the device is awake

src/sleep.cpp

@@ -14,6 +14,7 @@
 #include "esp_pm.h"
 #include "rom/rtc.h"
 #include <driver/rtc_io.h>
+#include <driver/uart.h>
 
 #include "BluetoothUtil.h"
 
@@ -111,7 +112,6 @@ void initDeepSleep()
 #endif
 }
 
-
 bool doPreflightSleep()
 {
     if (preflightSleep.notifyObservers(NULL) != 0)
@@ -257,6 +257,17 @@ esp_sleep_wakeup_cause_t doLightSleep(uint64_t sleepMsec) // FIXME, use a more r
     gpio_pullup_en((gpio_num_t)BUTTON_PIN);
 #endif
 
+#ifdef SERIAL0_RX_GPIO
+    // We treat the serial port as a GPIO for a fast/low power way of waking, if we see a rising edge that means
+    // someone started to send something
+
+    // Alas - doesn't work reliably, instead need to use the uart specific version (which burns a little power)
+    // FIXME: gpio 3 is RXD for serialport 0 on ESP32
+    // Send a few Z characters to wake the port
+    gpio_wakeup_enable((gpio_num_t)SERIAL0_RX_GPIO, GPIO_INTR_LOW_LEVEL);
+    // uart_set_wakeup_threshold(UART_NUM_0, 3);
+    // esp_sleep_enable_uart_wakeup(0);
+#endif
 #ifdef BUTTON_PIN
     gpio_wakeup_enable((gpio_num_t)BUTTON_PIN, GPIO_INTR_LOW_LEVEL); // when user presses, this button goes low
 #endif