As discussed in #1341, the current CC13xx/CC26xx IEEE mode driver sends `CMD_GET_RSSI` once and returns the RSSI reading uncondtionally. This happens within the `get_rssi()` function.
This logic is broken if `get_rssi()` is called with the radio off. The function will make sure to turn on the radio first, but it does not make sure the RSSI reading is valid, which only happens a number of symbol periods after the radio enters RX. The outcome is that `NETSTACK_RADIO.get_value(RADIO_PARAM_RSSI, ...)` will always return -128 (meaning that RSSI is unavailable) if the radio was off at the time of calling the function.
The same condition affects the prop mode driver.
This commit changes the logic of `get_rssi()`:
* For PROP mode, if `CMD_GET_RSSI` returns an invalid RSSI, we send it again. For unknown reasons, `CMD_GET_RSSI` on occasion returns 0, so we ignore that value too.
* For IEEE mode, we use `CMD_IEEE_CCA_REQ` and we inspect the value of `ccaInfo.ccaEnergy` of the return structure. If the value is 0x02 (Invalid), we send the command again.
Fixes#1341
After reading a frame, `read()` checks the status of the RX FIFO:
* If an overflow is detected, the FIFO gets flushed
* If there are more frames in the FIFO, the `cc2538_rf_process` will get polled again in order to read out the next frame.
#1550 changed `read()`, which now performs the above check for non-poll mode, but it then flushes the FIFO unconditionally. Therefore, if there are two or more frames in the FIFO, they will get flushed before the `cc2538_rf_process` has had a chance to read them out. This results in missed frames.
Reproducing this is trivial: Build a CC2538 sniffer and see how it will never show you .15.4 ACK frames. ACK reception completes while `read()` is still streaming the previous captured frame to the host. Upon completion, the FIFO will get flushed and the ACK will get lost.
This pull proposes removing the unconditional flush and reverting to the original logic for non-TSCH operation.
Pinging @thomas-ha here for input regarding poll mode.
This pull sits on top of #1778
#1684 slightly varied RF startup time and, as a result, threw ContikiMAC timing configuration off. This manifests itself with very poor CC13xx PROP mode radio performance, as documented in #1747.
This pull fixes this bug by slightly increasing ContikiMAC's inter-packet interval. This gives the receiver a little more time to respond with an ACK to unicast frames.
Fixes#1747
