Merge pull request ARMmbed#1844 from ARMmbed/fix_ack_tx_fail_handling

MAC: Fixed handling of the failed Ack transmission

source/MAC/IEEE802_15_4/mac_mcps_sap.c

@@ -1836,9 +1836,15 @@ static int8_t mcps_pd_data_cca_trig(protocol_interface_rf_mac_setup_s *rf_ptr, m
  }
  mac_pd_sap_set_phy_tx_time(rf_ptr, buffer->tx_time, cca_enabled);
  if (mac_plme_cca_req(rf_ptr) != 0) {
-
  if (buffer->fcf_dsn.frametype == MAC_FRAME_ACK) {
+ rf_ptr->mac_ack_tx_active = false;
+ // For Ack, stop the active TX process
  rf_ptr->macTxProcessActive = false;
+ // If MAC had TX process active before Ack transmission,
+ // the TX process has to be restarted in case the Ack transmission failed.
+ if (rf_ptr->active_pd_data_request) {
+ mac_csma_backoff_start(rf_ptr);
+ }
  return -1;
  }
  mac_csma_backoff_start(rf_ptr);