app: Change CTS pull-down to pull-up

Previously CTS was pulled up, this prevented hosts without
hardware flow control from using the default Serial Modem build.

Changing CTS to pull-down allows Serial Modem to send even
when the CTS and RTS pins float.

This relies in DTR as main mechanism for UART control. The host
must first enable its own UART before it enables Serial Modem
UART with DTR.

- With hardware flow control, this means that host drives it's
  RTS pin, so there is no difference in operation.
  Host should also have a pull-up for it's CTS pin, so when SM
  starts driving it's RTS, the host knows that SM is ready to
  receive.

- Without hardware flow control, the Serial Modem is able to
  send (TX) to host. However, the timing with DTR and when
  SM is able to receive needs to be taken into account as
  host cannot observe it's CTS. Instead host must monitor the
  RI signal, which is de-asserted when SM UART is ready.
  Or alternatively, have a sufficient timeout between host
  asserting DTR and host's first TX operation.

Important: It is highly recommended to use hardware flow control
with Serial Modem.

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>

Author: MarkusLassila

app/boards/nrf9151dk_nrf9151_ns.overlay

@@ -22,6 +22,10 @@
 		ri-gpios = <&gpio0 0 GPIO_ACTIVE_LOW>;
 		status = "okay";
 	};
+	pinctrl-0 = <&uart0_default_alt>;
+	pinctrl-1 = <&uart0_sleep_alt>;
+	pinctrl-names = "default", "sleep";
+
 };
 
 &gpio0 {
@@ -30,6 +34,34 @@
 	sense-edge-mask = <0x00000100>;
 };
 
+
+&pinctrl {
+	uart0_default_alt: uart0_default_alt {
+		group1 {
+			psels = <NRF_PSEL(UART_RX, 0, 26)>;
+			bias-pull-up;
+		};
+		group2 {
+			psels = <NRF_PSEL(UART_CTS, 0, 15)>;
+			bias-pull-down;
+		};
+		group3 {
+			psels = <NRF_PSEL(UART_TX, 0, 27)>,
+				<NRF_PSEL(UART_RTS, 0, 14)>;
+		};
+	};
+
+	uart0_sleep_alt: uart0_sleep_alt {
+		group1 {
+			psels = <NRF_PSEL(UART_TX, 0, 27)>,
+				<NRF_PSEL(UART_RX, 0, 26)>,
+				<NRF_PSEL(UART_RTS, 0, 14)>,
+				<NRF_PSEL(UART_CTS, 0, 15)>;
+			low-power-enable;
+		};
+	};
+};
+
 /* Disable I2C2.
  * - Enabling IO expander causes ~20uA to be consumed while DTR is deasserted.
  * - We use P0.31 and P0.30 pins for DTR and RI signals with external MCU.

app/overlay-external-mcu.overlay

@@ -45,11 +45,14 @@
 &pinctrl {
 	uart2_default_alt: uart2_default_alt {
 		group1 {
-			psels = <NRF_PSEL(UART_RX, 0, 3)>,
-				<NRF_PSEL(UART_CTS, 0, 7)>;
+			psels = <NRF_PSEL(UART_RX, 0, 3)>;
 			bias-pull-up;
 		};
 		group2 {
+			psels = <NRF_PSEL(UART_CTS, 0, 7)>;
+			bias-pull-down;
+		};
+		group3 {
 			psels = <NRF_PSEL(UART_TX, 0, 2)>,
 				<NRF_PSEL(UART_RTS, 0, 6)>;
 		};