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)>;
 		};

doc/uart_configuration.rst

@@ -71,11 +71,11 @@ The following tables shows how to connect the UART pins to the corresponding pin
          * - TX
            - P0.27
          * - RX
-           - P0.26
+           - P0.26 (pull-up)
          * - RTS
            - P0.14
          * - CTS
-           - P0.15
+           - P0.15 (pull-down)
          * - DTR
            - P0.08 (Button 1, pull-up, active high)
          * - RI
@@ -103,11 +103,11 @@ The following tables shows how to connect the UART pins to the corresponding pin
          * - TX
            - P0.02
          * - RX
-           - P0.03
+           - P0.03 (pull-up)
          * - RTS
            - P0.06
          * - CTS
-           - P0.07
+           - P0.07 (pull-down)
          * - DTR
            - P0.31 (active low, pull-up)
          * - RI
@@ -278,6 +278,13 @@ Once the host UART is ready and DTR is asserted, the nRF91 enables its UART inte
 The RI signal remains active throughout this process and only returns to inactive once the nRF91 UART is active.
 This coordinated wake-up ensures both ends of the communication link are ready before data transmission begins.
 
+**With hardware flow control:** The host can observe its CTS pin (connected to Serial Modem's RTS) to know when Serial Modem is ready to receive data.
+When Serial Modem starts driving its RTS pin, the host knows that Serial Modem is ready to receive.
+The host should also have a pull-up on its CTS pin.
+
+**Without hardware flow control:** The host must monitor the **RI signal**, which is de-asserted when the Serial Modem UART becomes ready to receive data.
+Alternatively, the host can implement a sufficient timeout between asserting DTR and initiating its first TX operation.
+
 Host-initiated wake-up
 ======================
 
@@ -289,6 +296,28 @@ The modem detects the DTR assertion and powers up its UART interface, allowing c
 
    Sequence diagram showing host-initiated wake-up of the modem
 
+.. _uart_without_flow_control:
+
+Operating without hardware flow control
+========================================
+
+.. important::
+   **It is highly recommended to use hardware flow control with Serial Modem** to ensure reliable communication and avoid timing-related issues.
+   Serial Modem has not been optimized for operation without hardware flow control.
+
+Serial Modem's CTS pin is configured with pull-down, allowing Serial Modem to transmit when CTS and RTS pins are left floating.
+The host can operate without connecting the hardware flow control signals (CTS/RTS).
+
+**Key requirements:**
+
+* The host must enable its own UART before asserting DTR
+* To detect when Serial Modem is ready to receive, monitor the RI signal (de-asserted when ready) or use a sufficient timeout after DTR assertion
+* See `Incoming data wake-up`_ for detailed timing information
+
+**Buffer configuration:**
+
+Increase ``CONFIG_SM_UART_RX_BUF_SIZE`` and possibly ``CONFIG_SM_UART_RX_BUF_COUNT`` to provide sufficient buffer space for your use case.
+
 Automatic UART power management using Zephyr's Cellular Modem driver
 ====================================================================