Live Telemetry

Note

MCU: PIC18F26K83 | RF Transceiver: TI CC1200 + GRF5509 PA

Overview

The LTT firmware runs on the RF telemetry board carried on the rocket and on the ground stations. It relays every RocketCAN message it sees outward over a sub-GHz RF link, and injects any messages it receives over RF back onto the local CAN bus. This gives mission control a redundant, wireless path to send commands (e.g. arming, actuator control) and receive telemetry while the rocket is railed or in flight, independent of the umbilical/CAN connection.

The same firmware image runs on three instance types, distinguished at build time by BOARD_INST_UNIQUE_ID:

ROCKET – lives on the rocket, acts as the network master
TELEMETRY_GROUND_1 / TELEMETRY_GROUND_2 – ground station radios

The ROCKET instance is the timing master: it round-robins the RF airtime between the ground instances so that only one radio transmits at a time (see RF Link / State Machine).

Reference Documents

PIC18F26K83 (MCU) Datasheet
CC1200 (RF Transceiver) Datasheet
CC1200 User’s Guide
SmartRF Studio – used to generate the register table in cc1200.c
GRF5509 (External PA) Datasheet

Initialization

board_Init() runs the following sequence before entering the main loop:

timer0_init() – start Timer0, which drives the millis() timebase (fires roughly every 500 us)
LED_Init() – configure Red/Green/Blue status LED pins
OSC_Init() – switch to the external HS oscillator, wait for ORDY
ADC_Init() – configure the ADC with the Fixed Voltage Reference (FVR, 2.048 V) and hardware low-pass filter, start continuous conversion on the current-sense input
SPI_Init() – configure the SPI peripheral used to talk to the CC1200
CAN_Init() – configure CAN TX/RX pins via PPS, compute bit timing from _XTAL_FREQ, initialize the canlib TX/RX buffer pools
CC1200_Init() – pulse the CC1200 RESET_n pin, configure the PA-enable pins as open-drain outputs, configure TMR3 to count received packets off the CC1200 PKT_CRC_OK GPIO, then write the full SmartRF-generated register table
SM_Init() – set the top-level state machine to LTT_STATE_INIT

The first pass through LTT_STATE_INIT (once the CC1200 confirms it has reached IDLE) additionally:

Loads the persisted operating frequency and TX power from EEPROM (eeprom_get_frequency() / eeprom_get_power()) and applies them to the CC1200
Enables antenna diversity on ground instances only (CC1200_Set_Ant_Diversity(!channel_is_rocket()))
Reports the active frequency/power back onto CAN via CONFIG_STATUS messages
Transitions to LTT_STATE_TX on the ROCKET instance, or LTT_STATE_RX on ground instances

Runtime

Heart Beat

The Blue LED toggles once per periodic CAN status report (see Health Check), i.e. approximately every 500 ms, as a liveness indicator.

The Green and Red LEDs are used as RF activity indicators rather than a fixed-period heartbeat:

Green – lit while this instance is transmitting (LTT_STATE_TX)
Red – lit while a message received over RF is being forwarded onto the local CAN bus

Health Check

Firmware samples the 12 V rail current every main-loop iteration through a 16 mOhm shunt and continuously filters it in software. The filtered value is reported every 500 ms via SENSOR_ANALOG.SENSOR_12V_CURR. If the reading is at or above the overcurrent threshold (200, in the ADC’s scaled mA units), the firmware sets the E_12V_OVER_CURRENT bit in GENERAL_BOARD_STATUS for that report period.
Firmware tracks RF link quality per remote instance: RSSI and LQI are accumulated for every packet received during an RX period and exponentially averaged when that period ends. If no packet has been received from a given remote within the last 3 seconds, that remote’s RSSI/LQI is reported as “no signal” (RSSI = -128, LQI = 0) rather than a stale value. This is reported every 500 ms via TELEMETRY_INFO, one message per known remote channel.

RF Link / State Machine

LTT communication is managed by two nested finite state machines.

Top-level FSM (SM_LTT_State_Machine) decides when this instance should transmit vs. receive:

INIT – apply persisted RF configuration (frequency, power, antenna diversity), then move to TX (ROCKET) or RX (ground)
TX – pop queued CAN messages and transmit them over RF; if no message has been sent for 50 ms (TX_TIMEOUT_MS), or the state has been active for 200 ms total (TX_TIME_MAX_MS), move to TX_END
TX_END – transmit a 1-byte end-of-transmission (EOT) frame naming the next instance that should transmit, then move to RX (or back to INIT if a configuration reload was requested)
RX – listen for packets; on ROCKET, if 60 ms (RX_TIMEOUT_MS) pass with nothing received, transmit anyway to guarantee telemetry keeps flowing even if ground receivers are too weak to reply. Non-ROCKET instances instead wait for an EOT frame addressed to their own instance ID before moving to TX. Any other RF packet received while in RX is pushed onto the local CAN TX queue and forwarded onto the bus.

The ROCKET instance is the only one that hands out transmit turns: it addresses its EOT frame to the next ground instance in round-robin order, so only one radio should ever be transmitting at a given moment. Ground instances only ever address their EOT frames back to ROCKET.

Low-level FSM (CC1200_State_Transition) drives the CC1200’s own RX/TX FIFOs based on chip status read via SNOP:

RX – decode any buffered packet, then fall through to IDLE handling
IDLE – if the top FSM wants to TX, pop a queued CAN message and write it to the TX FIFO (STX); if the top FSM wants RX, issue SRX
RX_FIFO_ERROR / TX_FIFO_ERROR – flush the offending FIFO (SFRX / SFTX)

Received-packet counting is done in hardware: TMR3 increments on every PKT_CRC_OK pulse from the CC1200, and the firmware compares that counter against its own bookkeeping to know whether unread packets are waiting in the RX FIFO, so a corrupted/dropped packet cannot desynchronize the two.

Frequency, Power, and Antenna Diversity Configuration

Operating frequency and TX power are stored in EEPROM so they survive reset/power-cycle, and are only re-applied to the CC1200 when the board (re-)enters LTT_STATE_INIT.
Both values can be changed remotely over CAN with CONFIG_SET (see CAN Communication); a successful write triggers SM_LTT_Reload_Config(), which causes the current TX/RX cycle to finish normally before the FSM returns to INIT and re-applies configuration – it does not interrupt an in-progress transmission.
Antenna diversity is enabled only on ground instances, since the ground stations are the ones equipped with the second, SRAD antenna path.
The Telemetry actuator command (ACTUATOR_TELEMETRY) can additionally:
- On ROCKET: turn the PA on/off (SM_LTT_Stop_TX), acting as a transmit inhibit independent of frequency/power configuration
- On ground instances: queue a one-shot telemetry “wake” packet (SM_LTT_Wake_Remote) that is transmitted the next time the RX timeout for that instance elapses without hearing from ROCKET – used to solicit a response from a rocket-side radio that has otherwise gone quiet

CAN Communication

CAN Message Sent by Firmware

CAN Message Sent by Firmware
Message Type	Description	Period
`SENSOR_ANALOG.SENSOR_12V_CURR`	Report filtered 12 V rail current	500 ms
`GENERAL_BOARD_STATUS`	Report board error bitfield (e.g. overcurrent)	500 ms
`TELEMETRY_INFO`	Report RSSI/LQI for one remote instance (one message per remote)	500 ms per remote
`CONFIG_STATUS` (`CAN_CONFIG_ID_FREQ`)	Report the currently-active RF frequency (offset from 900 MHz)	On entry to `LTT_STATE_INIT`
`CONFIG_STATUS` (`CAN_CONFIG_ID_POWER`)	Report the currently-active TX power (offset by +100)	On entry to `LTT_STATE_INIT`
(any CAN message queued locally)	Forwarded out over the RF link to the paired instance(s)	Best-effort, RF airtime permitting

CAN Message Handled by Firmware

CAN Message Handled by Firmware
Message Type	Description
`LEDS_ON`	Force all three status LEDs on (debug aid)
`LEDS_OFF`	Force all three status LEDs off (debug aid)
`ACTUATOR_CMD` (actuator = `ACTUATOR_TELEMETRY`)	`ON`: enable TX on ROCKET, or queue a wake packet on ground instances. `OFF`: disable TX (PA off) on ROCKET
`CONFIG_SET` (config id = 0 `CAN_CONFIG_ID_FREQ`)	Set persistent operating frequency (value is an offset from 900 MHz, in kHz) and request a config reload
`CONFIG_SET` (config id = 1 `CAN_CONFIG_ID_POWER`)	Set persistent TX power (value is offset by +100 to allow negative dBm) and request a config reload
`RESET_CMD`	Reset the board if targeted (`check_board_need_reset` in canlib)
(any other CAN message)	Queued for transmission out over the RF link

CONFIG_SET is only actioned if it targets BOARD_TYPE_ID_TELEMETRY and either this instance’s specific ID or BOARD_INST_ID_ANY.

GENERAL_BOARD_STATUS board specific error field usage

GENERAL_BOARD_STATUS board_error_bitfield Bit definition
Bitfield Name	Description	Offset
`E_12V_OVER_CURRENT`	12 V rail current at or above the overcurrent threshold	3