Servo - January 2013

conjunction with its laptop-based drivers — forms a virtual COM port. An LED on the PmodGPS blinks at 1 Hz to signal that a fix is yet to occur. A fix has been established when the LED extinguishes.

The PmodGPS’s 3DF pin drives the fix LED. The 3DF pin can also be used by a host microcontroller to determine fix status. Once a fix occurs, the NMEA strings flow from the PmodGPS into the PmodUSBUART which is displayed in a Tera Term session running on the laptop. According to the Electronics Explorer, the PmodGPS drew about 32 mA following a power-up cold start. Once a fix was acquired, the PmodGPS current consumption dropped to the level you see in Screenshot 1. The PmodGPS has no low-power mode. To conserve power, it must be powered off.

A coin cell holder resides on the opposite side of the PmodGPS’s circuit board. When the coin cell holder is populated with a battery, the PmodGPS can be powered down without losing any recent fix and time information. Without a coin cell in place, it would have to cold start with every power-up cycle. Cold starts typically take 35 seconds compared to one second for hot starts. If the PmodGPS is powered down for more than a couple of hours, a warm start is executed at power-up. Warm starts are 33 seconds in length.

Getting a Fix

The PmodGPS NMEA sentences begin with a dollar sign ($) character, which is followed by five characters of talker ID and arrival alarm. The letters GP comprise the talker ID. The remaining three characters identify the sentence output descriptor. All of the following fields of GPS-related information are comma delimited. NMEA sentences emitted from the PmodGPS look like this:

$GPGSA,M, 3,05, 17,04, 10, 12,02,,,,,,,1.60,1.28,0.96*0A $GPGSV, 2,1,08, 10, 69,062, 29,02, 56,321, 12,05, 47,208, 31, ,,,*4B

$GPRMC,171443.000,A,2821.8352,N,08040.3724,W,0.14, 159.70,231112,,,A*7E

$GPV TG,159.70, T,,M,0.14,N,0.25,K,A* 35

Let’s break down the RMC sentence ($GPRMC). The RMC sentence contains time, date, position, course, and speed data. The RMC sentence contents can be described as the recommended minimum navigation information. The first data field contains the UTC time formatted as hhmmss.sss. The A indicates that the data in the sentence is valid. Latitude information follows the validation character and is interpreted as ddmm.mmmm. Latitude is measured North or South of the Equator and the N denotes North. Longitude data is contained in the next comma-delimited field and is represented as dddmm.mmmm. Longitude associates East or West and that’s what the W tells us.

The PmodGPS on the bench is not in motion. So, the course and speed data really does nothing for us here. The next piece of valuable information is the date formatted as ddmmyy (231112). The final A defines the mode, which is autonomous. The sentence checksum is always preceded with a ‘*’ character and is always followed by a carriage return/line feed character sequence. The NMEA sentence structures are laid out in the PmodGPS reference manual.

Now that you have been introduced to the NMEA layout of an RMC sentence, you’ll notice that you can get the same time, latitude, and longitude data from the GGA (Global Positioning System Fix Data) sentence.

Acquiring the Data

$GPGGA,171443.000,2821.8352,N,08040.3724,W,1, 6,1.28

, 2. 2,M,- 31.1,M,,* 62

Knowledge of the structure of the NMEA sentences provides a basis for the code to acquire them. Each of the NMEA sentences contain less than 100 characters. At this point, we have plenty of microcontroller SRAM to spare. So, we can start by allocating five 128-character arrays. That’s one array for each sentence type:

char gpsbuf0[128];
char gpsbuf1[128];
char gpsbuf2[128];
char gpsbuf3[128];
char gpsbuf4[128];
BYTE bufcntr;

SCREENSHOT 1. The PmodGPS is based on the Global Top Gms-u1LP. The Gms-u1LP is fitted with an internal switch mode power supply that reduces power consumption by 30%.

40 SERVO 01.2013

The bufcntr byte will point at the buffer to be used for the next NMEA sentence. The idea is to constantly capture GPS sentence data and store it in the sentence buffers. A dollar character ($) delineates the beginning of every NMEA sentence. A carriage return/line feed sequence (0x0D, 0x0A) signals the end of every NMEA sentence.