18F252 GPS Questions n00b :)

TheRidDler · Joined: 13 Jun 2007 Posts: 7

Hi there all! Great forum! I have a couple of questions, please take it easy on me, im a total beginner and have scanned about every thread there is, trying to paste together some code to get my final year project up and running, but am unsure about just a couple of things and have decided to post...

Q1
I've got a 18F252 running at 3.6V and 20Mhz. Connected to the hardware UART of the pic is a LEA-4H GPS module. The GPS outputs are about 3V. I know the hardware UART is Schmitt triggered (0.8*Vdd = 2.88V) Just to make sure, will the GPS output be sufficient for data to be transferred?

Q2
I have a simple setup : I have to receive NMEA sentenced from a GPS (stated above) - and then send them via a RF transmitter to a PC. At the PC side im using the nRF905-433 RF tranceiver unit. At the PC side i only have to display the movement of the gps on a graph. I was thinking about extracting the GPRMC data from the sentences, and sending them. My question thus, will the 18F252 internal EEPROM be sufficient to store that one sentence prior to sending? If so, is there anyone that can help met with code to do the storage of the sentence in the internal EEPROM? Have browsed alot of threads but cant seem to get it right...

Thnx!

RossJ · Joined: 25 Aug 2004 Posts: 66

A1
The datasheet of your GPS module should tell you what the minimum logic high output voltage is. I expect you will find that it is lower than 2.88V. If so, and you are designing a product you need to work reliably, use a level converter to buffer the signal. If you are willing to risk intermittent problems, I have had a 3.3V radio module work 'reliably' with a PIC running at 5.0V (i.e. schmitt trigger threshold of 4.0V).

Are you transmitting to the GPS module, or just receiving position messages? If you are transmitting, have you considered the voltage difference in that direction?

A2
Why do you need to use EEPROM? Are sending the data over RF immediately, or do you need to hold the data for a period of time before dumping it to the PC in bulk? The only reason to use EEPROM is if you need the data to survive power cycles etc. Keep in mind that EEPROM has a limited number of write cycles, so you need to be clear on how frequently you intend to write, so you can ensure the chip doesn't fail prematurely.

Assuming you have a good reason to use EEPROM, have a look in the user guide for read_eeprom() and write_eeprom().

/Ross.

TheRidDler · Joined: 13 Jun 2007 Posts: 7

Hi Ross! Thnx for the reply.