Temperature on analogRead(38)

[mbustosorg]

Hi,

I'm having trouble getting a read from the on chip temp sensor. Is there any additional setup that has to be performed? I can't find anything in the datasheet or accompanying app note.

Thanks,
mauricio

 

[Jp3141]

Isn't it channel 26 (decimal) ? I think you interpreted 26 as hex and converted to decimal (-> 38) ?
Also, the result is not directly in degrees -- you have to calibrate each part, but this is start:

temperatureC = 432 - analogRead(temperatureChannel)*0.02936;

 

[mbustosorg]

Thanks for the response. I see the channel 26 but I also see 38 getting mapped to 26 in analog.c. What I think the problem was is that I wasn't properly calibrating. Working now!

Thanks again,
mauricio

static const uint8_t channel2sc1a[] = {
	5, 14, 8, 9, 13, 12, 6, 7, 15, 4,
	0, 19, 3, 21, 26, 22
};

int analogRead(uint8_t pin)
{
	int result;

	if (pin >= 14) {
		if (pin <= 23) {
			pin -= 14;  // 14-23 are A0-A9
		} else if (pin >= 34 && pin <= 39) {
			pin -= 24;  // 34-37 are A10-A13, 38 is temp sensor, 39 is vref
		} else {
			return 0;   // all others are invalid
		}
	}
	//serial_print("analogRead");
	//return 0;
	if (calibrating) wait_for_cal();
	//pin = 5; // PTD1/SE5b, pin 14, analog 0

	ADC0_SC1A = channel2sc1a[pin];
	while ((ADC0_SC1A & ADC_SC1_COCO) == 0) {
		yield();
		// wait
		//serial_print(".");
	}
	//serial_print("\n");
	result = ADC0_RA >> analog_right_shift;
	//serial_phex16(result >> 3);
	//serial_print("\n");
	return result;
}

 

[Jp3141]

It's physically channel 26 (dec) in the MCU; Paul's code maps this from channel 38 in an analogRead() call. It's just a (deliberate) coincidence that 0x26 = 38 decimal, so analogRead(0x26) actually works correctly !

 

[PaulStoffregen]

I just tried it with this....

[color=#CC6600]void[/color] [color=#CC6600][b]setup[/b][/color]() {
  [color=#CC6600]analogReference[/color]([color=#006699]INTERNAL[/color]);
  [color=#CC6600]analogReadResolution[/color](12);
  [color=#CC6600]analogReadAveraging[/color](32);
}

[color=#CC6600]void[/color] [color=#CC6600][b]loop[/b][/color]() {
  [color=#CC6600][b]Serial[/b][/color].[color=#CC6600]println[/color]([color=#CC6600]analogRead[/color](38));
  [color=#CC6600]delay[/color](10);
}

I'm running this right now, and it's printing 2391 and 2392. When I touched the edge of a cold bottle of water to the top of the chip, it went up to 2411 & 2412. When I touch the top of the chip with my soldering iron (in the center, not near any pins), it went down rapidly... somewhere around 2050 before I took the iron way after several seconds.

Of course this is an extremely imprecise test, but hopefully it helps?

 

[Jp3141]

Internally, the temperature sensor is just a diode (~ 0.7V at room temperature), and this falls at about -1.8 mV/°C. With 12 bits, a code of 2400 represents 2400/4096 * 1.2, or 0.7 V -- as you find (1.2 is approximately the internal reference value).

Each ADC code represents 1/4096*1.2 = 0.3 mV, or about 1/6 of a degree.

Freescale doesn't guarantee the room temperature value or temperature sensitivity, but it is quite easy to get << 1 degree resolution from this.