Servo - January 2013

PHOTO 3. This temperature probe can be subjected to harsh environments. The ENV-TMP's watertight construction allows it to swim. Thus, it’s perfect for monitoring the temperature of just about any liquid.

character and stuff every incoming byte into the selected sentence buffer until we encounter 0x0A.

If everything goes right, a single sentence will be placed in the targeted buffer. Once the sentence is captured, it can be identified by the three characters that follow the talker ID (GP). The fields are set for each sentence type, and we can simply parse the sentence to obtain the information we want to work with. I assembled the sentence buffer code using the C switch directive:

bufcntr = 0;
while(1){
 switch(bufcntr)
 {
case 0:
 i = 1;
 while(!(CharInQueue()));
 data_in = recvchar();
 if(data_in == ‘$’)
 {
 gpsbuf0[0] = data_in;
 do{
 data_in = recvchar();
 gpsbuf0[i++] = data_in;
 }while(data_in != 0x0A);
}
if(data_in == 0x0A)
{
bufcntr = 1;

 }
break;

Initially, the bufcntr variable is set to zero, which points to the 128-character array gpsbuf0. It is not necessary, but we will do it here. We’ll preserve the initial dollar sign character in the first element of each sentence array. That’s why you see the i index pointer initialized to 1 instead of 0.

The CharInQueue function blocks the execution of the program thread. However, the code that makes up the ISRs (interrupt service routines) is able to execute. Since we’re prototyping the GPS data acquisition function, at this time we’ll not write timer/iteration code around the CharInQueue call to force it to release and allow other code to execute. Plus, the PmodGPS is streaming in serial data to the PIC18F46J13’s USART every second.

Once the USART interrupt handler detects and buffers an incoming character, we use the recvchar function to pull a character from the tail of the USART_RxBuf array. If the character we pulled from the receive buffer array happens to be a dollar sign, we continually pull characters from the receive buffer until we encounter a linefeed character (0x0A).

Upon the arrival of the linefeed character, we set the bufcntr to point at the next gpsbufx array. We continually perform this process for each of the five gpsbufx arrays. Here’s the code for populating the second sentence buffer:

case 1:
 i = 1;
 while(!(CharInQueue()));
 data_in = recvchar();
 if(data_in == ‘$’)
 {
 gpsbuf1[0] = data_in;
 do{
 data_in = recvchar();
 gpsbuf1[i++] = data_in;
 }while(data_in != 0x0A);
 }
 if(data_in == 0x0A)
 {
bufcntr = 2;

 }
break;

A Rugged Robotic Temp Sensor

I normally employ the services of a Senserion temperature/humidity sensor for embedded applications. However, this is a robotic application. So, the temperature sensor I’ve selected is fit attire for a roving mechanical meteorologist. The business end of the Atlas Scientific ENV-TMP is posing for us in Photo 3.

The ENV-TMP is a breeze to use. All we have to do is supply power to the probe and pick up the temperature data with the microcontroller’s analog-to-digital converter (ADC). The red and black wires are the power connections. The ENV-TMP draws a mere 6 µA.

With current consumption that low, we could use one of the PIC’s I/O pins to power the temperature probe. The ENV-TMP’s white wire will feed our microcontroller’s 12-bit ADC. The temperature is gleaned from the ENV-TMP by converting the raw ADC count to millivolts.