MAC: Static CSMA period when using FHSS

This will prevent the synchronization error when too short CSMA random used.

source/MAC/IEEE802_15_4/mac_pd_sap.c

@@ -38,6 +38,10 @@
 /* Define TX Timeot Period */
 // Hardcoded to 1200ms. Should be changed dynamic: (FHSS) channel retries needs longer timeout
 #define NWKTX_TIMEOUT_PERIOD (1200*20)
+// Measured 3750us with 1280 byte secured packet from calculating TX time to starting CSMA timer on PHY.
+// Typically varies from 500us to several milliseconds depending on packet size and the platform.
+// MAC should learn and make this dynamic by sending first few packets with predefined CSMA period.
+#define MIN_FHSS_CSMA_PERIOD_US    4000
 
 static int8_t mac_data_interface_tx_done_cb(protocol_interface_rf_mac_setup_s *rf_ptr, phy_link_tx_status_e status, uint8_t cca_retry, uint8_t tx_retry);
 static void  mac_sap_cca_fail_cb(protocol_interface_rf_mac_setup_s *rf_ptr);
@@ -91,6 +95,13 @@ uint32_t mac_csma_backoff_get(protocol_interface_rf_mac_setup_s *rf_mac_setup)
     if (backoff_in_us == 0) {
         backoff_in_us = 1;
     }
+    if (rf_mac_setup->fhss_api) {
+        // Synchronization error when backoff time is shorter than allowed.
+        // TODO: Make this dynamic.
+        if (backoff_in_us < MIN_FHSS_CSMA_PERIOD_US) {
+            backoff_in_us += MIN_FHSS_CSMA_PERIOD_US;
+        }
+    }
     return backoff_in_us;
 }