NSSL EFM Instrument

Documentation for the NSSL EFM Instrument

Rotating Electronics

The rotating board is the primary data acquisition system for the instrument. It digitizes the analog output from the charge amplifier at high rate, reads a 9-DOF IMU and environmental sensor, tracks GPS position, and logs everything to a microSD card. Two record types are interleaved in the log: high-frequency ADC + IMU samples at 50 Hz and low-frequency GPS + environmental samples at 1 Hz.

Firmware Variants

The repository contains several firmware builds. Only original operational is the current flight firmware.

Directory	Purpose
`original operational`	Flight firmware — full sensor suite, SD card logging, GPS-synchronized timing
`nosd`	Development variant — SD removed; data output over serial to an external OpenLog
`nssl_custom_openlog`	Paired firmware for an OpenLog board used with the `nosd` variant
`simple bringup`	ADC-only bench test; used to verify ADS131M04 communication during hardware bring-up
`OpenLog_Custom`	Stripped SparkFun OpenLog firmware (no CLI, increased RX buffer) for use as a serial logger

For flight: use original operational.

System Overview

Component	Part	Interface
Microcontroller	STM32F103C8 (Blue Pill, 128 KB flash)	—
ADC	ADS131M04 (4-channel, 24-bit)	SPI
IMU	ICM-20948 (accel, gyro, magnetometer)	I2C
Environmental sensor	BME680 (temp, pressure, humidity)	I2C (addr 0x76)
GPS receiver	u-blox GNSS module	I2C + UART + PPS interrupt
Storage	microSD card	SPI
Status indicator	RGB LED	PWM (PB8, PB9, PA8)
Debug / radio port	UART	PB10/PB11
XBee port	UART	PA9/PA10

Compile-Time Flags

Define	Default	Description
`WAIT_ON_GPS_FIX`	defined	Block in `setup()` until GPS lock is acquired before beginning logging. Comment out for bench testing without GPS.
`WATCHDOG_ENABLE`	not defined	Enable the STM32 independent watchdog (26 s timeout). Should be defined for flight.
`DEBUG_PRINT_ENABLE`	not defined	Route verbose diagnostic messages to the debug UART (PB10/PB11) at 115200 baud.

For flight: define WATCHDOG_ENABLE. Leave WAIT_ON_GPS_FIX defined. Leave DEBUG_PRINT_ENABLE undefined.

Data Acquisition

ADC (ADS131M04)

The ADS131M04 is a 4-channel simultaneous-sampling 24-bit ADC. It is driven by interrupt on the DRDY line, which fires at 250 Hz. The ISR decimates by 5 to produce 50 Hz samples passed to the main loop.

Parameter	Value
Raw DRDY rate	250 Hz
Decimation factor	5
Effective sample rate	50 Hz
Oversampling ratio	OSR_16384
PGA gain (all channels)	1×
Input mux (all channels)	AIN0P / AIN0N (differential)

All four ADC channels are enabled and logged in every HF record, though in current hardware only some channels carry signals from the analog paddle.

IMU (ICM-20948)

The IMU is read once per 50 Hz ADC sample in the main loop. Gyroscope data is captured in the struct but currently commented out of the log output; only accelerometer and magnetometer are written to the log.

Sensor	Logged	Units in log
Accelerometer (X, Y, Z)	Yes	mg (×1000 of g)
Gyroscope (X, Y, Z)	No (captured, not written)	—
Magnetometer (X, Y, Z)	Yes	µT (1× scale, integer)

Environmental Sensor (BME680)

Sampled once per second alongside GPS. Gas heater is disabled; only temperature, pressure, and humidity are used.

Channel	Oversampling	Units in log
Temperature	8×	°C × 10 (int16)
Pressure	4×	hPa × 10 (int16)
Humidity	2×	%RH × 10 (int16)

A failed BME read writes INT16_MIN (−32768) to all three fields.

GPS and Timing

The u-blox receiver is configured at startup (settings saved to battery-backed memory):

Setting	Value
Dynamic model	AIRBORNE <2g>
Navigation frequency	1 Hz
UART1 baud rate	115200
Enabled NMEA sentences	RMC, ZDA, GGA
Time pulse (PPS)	1 Hz, 100 ms active-high pulse

GPS initialization begins with a factory reset (factoryReset() + factoryDefault()), so settings are re-applied on every boot.

PPS Synchronization

The PPS rising edge fires an ISR (ppsISR) which captures millis() and resets the 50 Hz sample index counter (hfIdx = 0). Each HF record stores:

pps_utc — the raw GPS time value from TinyGPS++ at the last valid sentence
ms_offset — which 20 ms grid slot within the current second the sample belongs to (idx × 20)
processor_ms — the raw millis() value when the record was built

Log File Format

File Naming

Log files are named MMDDHHMM.TXT based on GPS time, where the minute is rounded down to the nearest 10-minute boundary. A new file is opened every 10 minutes. Each file is pre-allocated to 4 MB on creation to reduce SD fragmentation. If GPS lock has not yet been acquired the file is named 00000000.TXT.

Record Types

Two record types are interleaved in each file, identified by the first field.

HF Record (type `1`) — 50 Hz

1,<pps_utc>,<ms_offset>,<processor_ms>,<ch0>,<ch1>,<ch2>,<ch3>,<ax>,<ay>,<az>,<mx>,<my>,<mz>

Field	Type	Description
`1`	literal	Record type identifier
`pps_utc`	uint32	GPS UTC time value from TinyGPS++ (format: HHMMSSCC)
`ms_offset`	uint32	Milliseconds since last PPS (0, 20, 40, … 980)
`processor_ms`	uint32	`millis()` at time of record
`ch0`–`ch3`	uint32	Raw ADS131M04 24-bit ADC counts (4 channels)
`ax`, `ay`, `az`	int16	Accelerometer in mg (g × 1000)
`mx`, `my`, `mz`	int16	Magnetometer in µT (integer)

LF Record (type `2`) — 1 Hz

2,<pps_utc>,<ms_offset>,<processor_ms>,<lat>,<lon>,<alt>,<temp>,<pressure>,<humidity>

Field	Type	Description
`2`	literal	Record type identifier
`pps_utc`	uint32	GPS UTC time value from TinyGPS++
`ms_offset`	uint32	Milliseconds since last PPS
`processor_ms`	uint32	`millis()` at time of record
`lat`	int32	Latitude × 10⁵ (decimal degrees)
`lon`	int32	Longitude × 10⁵ (decimal degrees)
`alt`	uint16	Altitude in whole metres (rounded)
`temp`	int16	Temperature in °C × 10 (`INT16_MIN` on error)
`pressure`	int16	Pressure in hPa × 10 (`INT16_MIN` on error)
`humidity`	int16	Humidity in %RH × 10 (`INT16_MIN` on error)

Write Behavior

The circular log buffer holds up to 128 LogEntry records. flushLog() drains up to 32 entries per call and flushes to the SD card at ~10 Hz. GPS is polled every 8 records during drain to avoid starving the NMEA parser.

LED Status Indicators

Color	Meaning
RGB cycle on power-up	Normal boot sequence (red → green → blue → off, 1 s each)
Blue (solid)	Waiting for GPS lock
Green (solid)	Logging normally
Red (blinking)	Setup failed — sensor init or SD card error
Red (solid)	Runtime error — SD write failure

Pin Assignments

Pin	Name	Direction	Description
PA0	`PIN_ADC_CS`	Output	ADS131M04 SPI chip select
PA1	`PIN_GPS_PPS`	Input	GPS 1PPS interrupt
PA2	`PIN_GPS_RX`	RX	GPS UART RX (SerGPS)
PA3	`PIN_GPS_TX`	TX	GPS UART TX (SerGPS)
PA4	`PIN_ADC_DRDY`	Input	ADS131M04 data-ready interrupt
PA5	`PIN_SPI1_SCLK`	Output	SPI clock (ADC + SD)
PA6	`PIN_SPI1_MISO`	Input	SPI MISO
PA7	`PIN_SPI1_MOSI`	Output	SPI MOSI
PA8	`PIN_LED_BLUE`	Output	Blue LED (PWM)
PA9	`PIN_XBEE_TX`	TX	XBee UART TX
PA10	`PIN_XBEE_RX`	RX	XBee UART RX
PA15	`PIN_ADC_RESET`	Output	ADS131M04 hardware reset
PB4	`PIN_SD_CD`	Input	SD card-detect
PB5	`PIN_SD_CS`	Output	SD card SPI chip select
PB6	`PIN_I2C_SCL`	Output	I2C clock (IMU, BME680, GPS)
PB7	`PIN_I2C_SDA`	I/O	I2C data
PB8	`PIN_LED_RED`	Output	Red LED (PWM)
PB9	`PIN_LED_GREEN`	Output	Green LED (PWM)
PB10	`PIN_RADIO_RX`	RX	Debug / radio UART RX
PB11	`PIN_RADIO_TX`	TX	Debug / radio UART TX

Firmware Checkout Procedure

Power-On Self-Test

Step	Expected Outcome
Apply power	LED cycles red → green → blue → off (1 s each)
LED turns blue after boot sequence	Waiting for GPS lock — all sensors initialized correctly
LED turns red and blinks rapidly	Setup failed — check sensor connections and SD card

Sensor Initialization Verification

Enable DEBUG_PRINT_ENABLE and connect to the debug UART (PB10/PB11) at 115200 baud to see per-sensor init results during boot.

Check	Expected Debug Output
ADC	`Initializing ADC... ok`
IMU	`Initializing IMU... ok`
BME680	`Initializing BME688... ok`
GPS	`Initializing GPS... ok`
SD init	`Initializing SD card... ok`
SD write test	`Testing SD write... ok`

GPS Lock and Logging

Step	Expected Outcome
Wait for GPS fix (outdoors or with antenna)	LED turns green
Remove SD card and inspect file listing	File named `MMDDHHMM.TXT` present (or `00000000.TXT` if no lock)
Open log file in text editor	Mix of lines starting with `1,` and `2,`
Count `1,` lines per second	Should be approximately 50
Count `2,` lines per second	Should be approximately 1

Data Quality Checks

Check	Expected Outcome
`alt` field in LF records	Plausible altitude in metres for the location
`temp` / `pressure` / `humidity`	Not `−32768` (which indicates a BME read failure)
`ax`, `ay`, `az` in HF records with unit at rest	One axis near ±1000 (1 g), other two near 0
`ms_offset` sequence in HF records	Steps of 20 ms (0, 20, 40, … 980), then resets
`pps_utc` advances by 1 second per group of 50 HF records	Confirms PPS synchronization

File Rollover Verification

Step	Expected Outcome
Allow unit to log across a 10-minute boundary	New `MMDDHHMM.TXT` file created
Inspect previous file	Closed cleanly; no truncated records
Each file pre-allocated to 4 MB	Visible as file size on SD; unfilled space is zeroed

Cleanup

After testing with DEBUG_PRINT_ENABLE or WATCHDOG_ENABLE disabled, reflash the flight build with both flags in their correct flight state before deployment.