samples: net: lwm2m: Add support for nRF52840 on nRF9160DK

Add support for running LwM2M client on nRF52840 chip
using nRF9160 as a modem through serial port.
nRF9160 should run Serial LTE Modem application build
from nRF Connect SDK.

nRF9160DK board has two MCU, nRF9160 and nRF52840.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

samples/net/lwm2m_client/Kconfig

@@ -27,10 +27,14 @@ config LWM2M_APP_TLS_TAG
 	  TLS security tag to use when connecting.
 endif
 
+config HAS_INTERNET
+	bool
+	default y if (WIFI || MODEM)
+
 config LWM2M_APP_SERVER
 	string "LwM2M server address"
-	default "coap://leshan.eclipseprojects.io:5683" if (WIFI && !LWM2M_DTLS_SUPPORT)
-	default "coaps://leshan.eclipseprojects.io:5684" if (WIFI && LWM2M_DTLS_SUPPORT)
+	default "coap://leshan.eclipseprojects.io:5683" if (HAS_INTERNET && !LWM2M_DTLS_SUPPORT)
+	default "coaps://leshan.eclipseprojects.io:5684" if (HAS_INTERNET && LWM2M_DTLS_SUPPORT)
 	default "coap://192.0.2.2:5683" if !LWM2M_DTLS_SUPPORT
 	default "coaps://192.0.2.2:5684" if (LWM2M_DTLS_SUPPORT && !LWM2M_RD_CLIENT_SUPPORT_BOOTSTRAP)
 	default "coaps://192.0.2.2:5784" if (LWM2M_DTLS_SUPPORT && LWM2M_RD_CLIENT_SUPPORT_BOOTSTRAP)

samples/net/lwm2m_client/boards/nrf9160dk_nrf52840.conf

@@ -0,0 +1,34 @@
+CONFIG_UART_ASYNC_API=y
+CONFIG_UART_1_ASYNC=y
+CONFIG_UART_1_INTERRUPT_DRIVEN=n
+
+# Align with the Serial LTE Modem (SLM) application.
+CONFIG_MODEM_CELLULAR_UART_BUFFER_SIZES=6000
+
+# Networking
+CONFIG_NETWORKING=y
+CONFIG_NET_NATIVE=y
+CONFIG_NET_L2_PPP=y
+CONFIG_NET_IPV4=y
+CONFIG_NET_UDP=y
+CONFIG_NET_SOCKETS=y
+CONFIG_NET_CONTEXT_RCVTIMEO=y
+
+# DNS
+CONFIG_DNS_RESOLVER=y
+CONFIG_NET_L2_PPP_OPTION_DNS_USE=y
+CONFIG_DNS_SERVER_IP_ADDRESSES=y
+CONFIG_DNS_SERVER1="8.8.8.8"
+CONFIG_LWM2M_DNS_SUPPORT=y
+
+# Network management
+CONFIG_NET_MGMT=y
+CONFIG_NET_MGMT_EVENT=y
+CONFIG_NET_CONNECTION_MANAGER=y
+
+# Modem driver
+CONFIG_MODEM=y
+CONFIG_MODEM_CELLULAR=y
+
+# LwM2M client does not use power management
+CONFIG_PM_DEVICE=n

samples/net/lwm2m_client/boards/nrf9160dk_nrf52840.overlay

@@ -0,0 +1,18 @@
+#include <nrf52840/nrf9160dk_uart1_on_if0_3.dtsi>
+
+/ {
+	aliases {
+		modem = &modem;
+	};
+};
+
+&uart1 {
+	current-speed = <115200>;
+	hw-flow-control;
+
+	modem: modem {
+		compatible = "nordic,nrf91-slm";
+		status = "okay";
+		mdm-power-gpios = <&interface_to_nrf9160 4 GPIO_ACTIVE_LOW>;
+	};
+};

samples/net/lwm2m_client/src/lwm2m-client.c

@@ -376,6 +376,13 @@ int main(void)
 	k_sem_init(&quit_lock, 0, K_SEM_MAX_LIMIT);
 
 	if (IS_ENABLED(CONFIG_NET_CONNECTION_MANAGER)) {
+		struct net_if *iface = net_if_get_default();
+
+		if (!iface) {
+			LOG_ERR("No network interface found!");
+			return -ENODEV;
+		}
+
 		/* Setup handler for Zephyr NET Connection Manager events. */
 		net_mgmt_init_event_callback(&l4_cb, l4_event_handler, L4_EVENT_MASK);
 		net_mgmt_add_event_callback(&l4_cb);
@@ -385,14 +392,14 @@ int main(void)
 					     CONN_LAYER_EVENT_MASK);
 		net_mgmt_add_event_callback(&conn_cb);
 
-		ret = net_if_up(net_if_get_default());
+		ret = net_if_up(iface);
 
 		if (ret < 0 && ret != -EALREADY) {
 			LOG_ERR("net_if_up, error: %d", ret);
 			return ret;
 		}
 
-		(void)conn_mgr_if_connect(net_if_get_default());
+		(void)conn_mgr_if_connect(iface);
 
 		LOG_INF("Waiting for network connection...");
 		k_sem_take(&network_connected_sem, K_FOREVER);