Thank you for the suggestion! It is going to be used as part of a tag collecting oxygen data in marine environments. The tag has two oxygen sensors and a speed sensor that need to be able to log data onto the board.

Hey Andreas, can an Arduino actually keep the system clock in sync with the GPS time on a continuous basis? Because on the Raspberry Pi there's quite a sophisticated software stack to do this involving a GPS daemon, shared memory and a complex chrony time daemon making micro adjustments to avoid stepping the clock.

I would expect that an arduino would not support this though I expect that it is likely that it would allow you to "step" the clock if you chose to. This would result jumps in the actual time. I would imagine that there is an easy way to get relatively good time from the GPS to associated with your data in a polled manner however.

Actually, if it's only used three days at a time then maybe it's also an option to use a real time clock chip and simply sync it before going out with the sensor. I think it would only have a very small amount of drift in a day.

Hello, 

Thank you for the response, I apologize I was out of town. I will be connecting an external battery to the board to power the sensors, so I should have power available. I would have to look into the requirements of a Raspberry Pi. The tag will only be deployed for 72 hours at a time, and the power draw of the sensors is minimal (10mA) each. 

If the data logger is located above water with a view of the sky, then a Raspberry Pi zero can be made to last 3 days on a battery. I got about 3 days on a poor quality 50,000 mha lipo battery when recording and writing audio continuously.

The Pi with a GPS can be made to maintain sub-microsecond system clock accuracy by installing my sbts-aru project and then just disabling the audio recorder. Then it just runs the system time sync functionality. It also turns off unnecessary services to keep power usage low.

It then installs a GPS time synced base with SD resilience (With a memory overlayFS) in one command, which is always good when working with Raspberry Pi and SD cards.

The Pi when powered by a 5v battery will provide a 3V output that can be used by other sensors. If the Pi is to be located below water, then you could still in a water proof way have an external GPS antenna to bring this above the water.

The analog input would likely have to be an addon. Maybe there are simple i2c sensors that provide this. Here's an article about this this sort of thing.

Requiring two uarts at least gets harder if using a Raspberry Pi zero. One would be used by a GPS if using that for time as well, I think you only have one left over then.

I think the new Raspberry Pi 5 has a bunch more uarts available, but then the power usage would be a lot higher. You could also use more than one Raspberry Pi zero, each with their own battery and and have two spare uarts over for sensors, one synching it's time from the other over wifi, then you have two uarts on that pi.

Hi Kim, 

Thank you for the suggestions. I think I misunderstood, the built in clock is just so that I can match up timestamps between the sensors and a separate accelerometer. The purpose of the clock is so I can coordinate the data between the two. The boards themselves will be in a housing underwater. The previous data logger I was using would lose time if it was powered down after being set via computer.

Oh okay. That super simply solved by attaching a real time clock module on a Raspberry Pi, they have a battery to keep the time going. Must be similar ones for microcontrollers if you go that route.

Thank you for your help!

You could prototype on an Arduino and, if necessary, implement the final version directly using the AVR chips on which most of the Arduinos are based. The AVR chips have deep sleep modes in which only the clock is running. It's very easy to build circuits with those chips. If you only need to pass ~10mA of current (at around 3-5v) to a few sensors then you might be able to do it without additional hardware. 

So which model microcontroller has at least two uarts ?

If it's a standard Arduino Uno, you can use the Software Serial library to emulate the serial ports you need. Otherwise you can use the ATMega1284P and the MightyCore core files which opens things up to two hardware serial ports (UARTs) as well as software serial. Or you can use the ATMega4809 with MegaCoreX which will give you up to 4 hardware serial ports (1 for debug and up to 3 for peripherals).  These are all usable within the Arduino environment. 

Good luck with your project!

Akiba

Great summary, thanks Akiba.

Thank you all for this advice! I'll be looking into all of these options. I really appreciate you all taking the time to think about this.

I ran out of time to pull this together for my current research trip, but I hope to be able to implement it for my summer field season.