Posts Tagged ‘MTK3339’

Using Ultimate GPS to bootstrap Maxim DS3231M

Wednesday, June 11th, 2014

I’m currently building a project that is based around a Maxim DS3231M, which is an I2C-attached RTC.

I’m currently using the chip through the TinyTempus breakout from Phenoptix which only lacks the 32.768 KHz output which I don’t use anyway. I’m using the provided CR1216 battery to keep the clock ticking at all times however I had a need to get the clock set to the right time.

As I had an Adafruit Ultimate GPS breakout I figured it would be easy to just use the time from that to bootstrap the clock to the right time rather than deal with user input. I put together the following basic circuit:


I’ve uploaded the sketch I wrote to GitHub:

The sketch uses the undocumented $GPZDA NMEA sentence to retrieve the time from the GPS module and reprograms the DS3231 on receipt of every new sentence so you just need to run the sketch long enough to get the message that the clock is set. Assuming your battery is good, the clock will keep ticking and maintain time accurately after you pull the power.

Enabling $GPZDA NMEA sentence on Adafruit Ultimate GPS

Tuesday, June 10th, 2014

I’ve been using an Adafruit Ultimate GPS module as a time source which with their standard GPS Library provides the $GPRMC NMEA 0183 sentence containing the time and date however I had a preference for the $GPZDA sentence for the simple fact it provides the year as four digits instead of only two.

This forum post suggested it was possible on the MTK3339 chip by sending an unsupported variant of the command that enables the various sentences:


However, this didn’t work. The datasheet provided by Adafruit doesn’t document the majority of the data fields in this command, just identifying them as reserved. I found another datasheet elsewhere which I think is just an older revision of the same document and this has slightly better information. In fact, the command was only slightly wrong, it’s in fact this:


It’s the 18th field, not the 17th as the forum post originally suggested. Sending this command to the GPS module now returns the $GPZDA NMEA sentence. It can be enabled with other sentences, you just need to update the XOR checksum after the * using something like this online tool.