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SIM33EAU GPS Module -- Helpful tips — Parallax Forums

SIM33EAU GPS Module -- Helpful tips

  1. Download and print the Parallax document, "SIM33EAU GPS Module (#28504) Rev C." Go to page 6 and read the Quick-Start Guide. (Available on the Parallax SIM33EAU GPS Module (#28504) product page.)

  2. If you are starting a first project with a SIM33EUA Module, download and print the document, "MT3337 Platform NMEA Message Specification_V1.00." It is a helpful reference. (Also available on the Parallax SIM33EAU GPS Module (#28504) product page.) It's a good reference for other GPS modules, too, because the information conforms to a NMEA standard.

  3. The SIM33EAU GPS Module (module) does not need any initialization to start. Power it and connect its serial output to an MCU board with a serial port set to 9600 bits/sec. Route the data to the Parallax Serial Terminal (PST) or to another terminal that accepts 5V--3V logic inputs. The output is NOT RS-232C. As soon as the module has enough satellite "fixes" (about half a minute for a cold start) to determine a position, you'll see useful data on the terminal.

  4. You can configure the module for many types of "sentences" that provide different information. The RMC sentence is a good one to start with. To program the module to transmit ONLY this message you must send it a sequence of characters such as:


via the module's RXD input pin. A PMTK-type message always starts with a dollar sign ($) and ends with an asterisk (*). the values after the asterisk are a checksum value that the module also calculates and compares with the one at the end of your message. The message ends with a carriage-return (0x0D) and a line-feed value (0x0A).

To calculate the proper checksum, go to the web site: Insert the information you want to send between the $ and the * and the calculator produces the message complete with the $, *, and checksum. It's a nice tool.

  1. For information about the PMTK314 message, refer to page 16 in document noted in section 2 above. The PMTK314 example message is poorly documented. Here's a complete explanation:


The first five "1" values cause the module to transmit the following messages at 1-second intervals:


The The value "5" sets the NMEA_SEN_GSV sentence to be transmitted only at five-second intervals.

Sorry, but I have no idea what the last two 1's represent, other than what the document explains thus:

NMEA_SEN_MCHN, //PMTKCHN interval – GNSS channel status
NMEA_SEN_DTM, //GPDTM interval – Datum reference

You could leave them as shown above in the PMTK message and see what happens. IMPORTANT: If you change the information in a PMTK "packet," ALWAYS calculate a new checksum!

  1. You may change the timing of data output (default is 1x/sec) via a different PMTK message. Refer to page 13 in the document referred to in section 2 above.

  2. Bit rate setting. The message:


for example, changes the data rate to 4800 bits/sec. Note the calculated checksum of 14 at the end before the CR and LF. Use the online checksum calculator. The checksum is sent as two characters, a 1 (0x31) and a 4 (0x34). IMPORTANT: It is NOT sent at 0x14.

  1. In some GPS modules any changes you make to the default settings and bit rate return to the default values when the module loses power. On the SIM33EAU GPS Module a super-capacitor provides about 2 hours of backup power. After that you might need to reprogram your settings when the GPS module again gets powered. Someone can check this for the Parallax module.

Feel free to add your observations, comments or corrections. --Jon

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