We are presented with an embedded tracking device and we need to extract an exact location at a specific time. By looking at the provided electronic scheme, we can see that we have the following components:
- Atmel ATmega2560 - Embedded micro controller. Datasheet
- u-blox SAM-M8Q - GPS reciever - UART output (NMEA)
- ADXL345 - Accelerometer, I2C output with interrupt line
- Macronix MX66L1G45G - SPI serial NOR flash
By examining the provided code we can understand the following characteristics of the tracking device:
- On startup it configures all the peripherals
- It configures a timer interrupt that reoccurrs in every ?? (Need to figure out that)
- It waits for a specific GPS NMEA message (GPRMC), parses it, extracts the current date & time and save it to the flash.
- Whenever suitable, it reads GPS NMEA message (GPGGA) and write the coordinates to the flash.
- An accelerometer interrupt is used to change between states, probably the accelerometer triggers interrupt on activity and inactivity.
- Messages are saved to flash in TLV (Tag, Length, Value) encoding.
My first step was to write a reader that reads the saved flash, basically just go over the entire file and print the data. After doing that, I was able to extract the coordinates, but I couldn't tell when each was taken.
I needed to know how often the timer interrupt occurs.
Searching for the datasheet for the Atmel ATmega2560 and reading about the timer interrupt I was able to understand how
the timer period is calculated:
All the formula parameters are extracted from program.c source code.
I calculated the timer frequency to be 4 sec, this means, that the function interrupt functions is called once every 4 sec. In the function it self, it increments a counter and if it's equal to counter_max_val it sets is_triggered to true. counter_max_val can be set to either 75 (in startup) 15 or 150. This means that is_triggered will be set to true once in every 5 minutes, 1 minute or or 10 minutes.
Whenever there's an accelerometer event that changes counter_max_val, it saves that event to the flash. So basically, if we know the start time, and the current state, we can track the time where each GPS measurement was taken.
The C program main.c implements a state machine that changes state while printing out the coordinates & time. You can run it, and if you care for the answer grep the output so it only prints the time "01:21"
Latitude: 32.21893, Longitude: 35.23226 Tue Oct 30 01:21:00 2018
That's it, enter the latitude and longitude coordinates and you can start the next challenge.
- You can read more on timer interrupts here
- NMEA messages format