cdc_uart: add CTS/RTS configuration options to board_example_config

For high data rate applications it's desirable to use hardware flow control
to prevent characters getting dropped when faced with the vagaries of RTOS
and kernel latencies. Adding PROBE_UART_HWFC enables the UART's CTS/RTS pins,
and SET_LINE_STATE messages no longer affect the RTS pin.

include/board_example_config.h

diff --git a/include/board_example_config.h b/include/board_example_config.h
index 825e6fb..517916b 100644
--- a/include/board_example_config.h
+++ b/include/board_example_config.h
@@ -37,6 +37,10 @@
 
 /* Include CDC interface to bridge to target UART. Omit if not used. */
 #define PROBE_CDC_UART
+
+/* Board implements hardware flow control for UART RTS/CTS instead of ACM control */
+#define PROBE_UART_HWFC
+
 /* Target reset GPIO (active-low). Omit if not used.*/
 #define PROBE_PIN_RESET 1
 
@@ -68,9 +72,17 @@
 #define PROBE_UART_RX 5
 #define PROBE_UART_INTERFACE uart1
 #define PROBE_UART_BAUDRATE 115200
-/* Flow control - some or all of these can be omitted if not used */
+
+#if defined(PROBE_UART_HWFC)
+/* Hardware flow control - see 1.4.3 in the RP2040 datasheet for valid pin settings */
+#define PROBE_UART_CTS 6
+#define PROBE_UART_RTS 7
+#else
+/* Software flow control - RTS and DTR can be omitted if not used */
 #define PROBE_UART_RTS 9
+#endif
 #define PROBE_UART_DTR 10
+
 #endif
 
 /* LED config - some or all of these can be omitted if not used */

src/cdc_uart.c

diff --git a/src/cdc_uart.c b/src/cdc_uart.c
index 6379aad..5c79781 100644
--- a/src/cdc_uart.c
+++ b/src/cdc_uart.c
@@ -57,11 +57,23 @@
     gpio_set_pulls(PROBE_UART_RX, 1, 0);
     uart_init(PROBE_UART_INTERFACE, PROBE_UART_BAUDRATE);
 
+#ifdef PROBE_UART_HWFC
+    /* HWFC implies that hardware flow control is implemented and the
+     * UART operates in "full-duplex" mode (See USB CDC PSTN120 6.3.12).
+     * Default to pulling in the active direction, so an unconnected CTS
+     * behaves the same as if CTS were not enabled. */
+    gpio_set_pulls(PROBE_UART_CTS, 0, 1);
+    gpio_set_function(PROBE_UART_RTS, GPIO_FUNC_UART);
+    gpio_set_function(PROBE_UART_CTS, GPIO_FUNC_UART);
+    uart_set_hw_flow(PROBE_UART_INTERFACE, true, true);
+#else
 #ifdef PROBE_UART_RTS
     gpio_init(PROBE_UART_RTS);
     gpio_set_dir(PROBE_UART_RTS, GPIO_OUT);
     gpio_put(PROBE_UART_RTS, 1);
 #endif
+#endif
+
 #ifdef PROBE_UART_DTR
     gpio_init(PROBE_UART_DTR);
     gpio_set_dir(PROBE_UART_DTR, GPIO_OUT);