BeagleBone – How hot is it?

TMP36 Temperature Sensor

I picked up a TMP36 – Analog Temperature sensor from adafruit. Time to see how to connect it to a my BeagleBone.

This sensor provides a voltage output that is linearly proportional to the Celsius (centigrade) temperature.

The TMP36 is specified from −40°C to +125°C, provides a 750 mV output at 25°C, and operates to 125°C from a single 2.7 V supply.

You calculate the temperature with the following forumula:

Temperature = ((Vout in mV) – 500mV) / 10.

I can use one of the BeagleBone’s analog to digital (ADC) interfaces to read the ouput from the TMP36. Things to get a little complicated because the ADC inputs are only 1.8V interfaces and the TMP36 is a 2.7V interface. If the temperature got hot enough the TMP36 could push enough voltage to potentially fry the ADC input. Since things won’t get that hot I’ll not worry about it. My tests won’t get an output from the TMP36 anywhere near 1.8V.

The TMP36 will be powered by pin 3 (3.3V) on expansion header P9. Ground can go to any of the ground pins on the expansion headers. I’ll use pin 1 on expansion header P8 for ground. The analog output will go to ADC interface AIN1 – pin 40 on expansion header P9. The BeagleBone’s ADC interfaces return a 12 bit value which will range from 0 to 4095.

BeagleBone with Temperature Sensor

Programming will again use the Linux’s sysfs. /sys/devices/platform/tsc provides access to the ADC interfaces. There are eight AIN files representing the eight ADC interfaces. To make things interesting their number scheme starts at 1 where as the symbolic name in the BeagleBone manual start at 0.

SIGNAL NAME	AIN File Name
AIN0	/sys/devices/platform/tsc/ain1
AIN1	/sys/devices/platform/tsc/ain2
AIN2	/sys/devices/platform/tsc/ain3
AIN3	/sys/devices/platform/tsc/ain4
AIN4	/sys/devices/platform/tsc/ain5
AIN5	/sys/devices/platform/tsc/ain6
AIN6	/sys/devices/platform/tsc/ain7
AIN7	/sys/devices/platform/tsc/ain8

I can easily read the digital value of AIN1 from the shell using:

#cat /sys/devices/platform/tsc/ain2
1670

In this case I got back a value of 1670. Using a simple formula I can convert that value to the temperature.

Step	Description	Example
1	Read the digital value from the ADC interface.	#cat /sys/devices/platform/tsc/ain2 1670
2	Convert the digital value to millivolts. (value / 4096) * 1800mV	(1670 / 4096) * 1800mV = 733.8867mV
3	Convert the millivolts to Celsius temperature. (millivolts – 500mV) / 10	(733.8867mV – 500mv) / 10 = 23.38867°C
4	Convert Celsius to Fahrenheit. (Celsius * 9.0 / 5.0) + 32.0	(23.38867°C * 9 / 5) + 32 = 74.09961°F

Time for some C code.

#include
#include

double CtoF(double c) {
        return (c * 9.0 / 5.0) + 32.0;
}

double temperature(char *string) {
        int value = atoi(string);
        double millivolts = (value / 4096.0) * 1800;
        double temperature = (millivolts - 500.0) / 10.0;
        return temperature;
}

void main() {
        int fd = open("/sys/devices/platform/tsc/ain2", O_RDONLY);

        while (1) {
                char buffer[1024];
                int ret = read(fd, buffer, sizeof(buffer));
                if (ret != -1) {
                        buffer[ret] = NULL;
                        double celsius = temperature(buffer);
                        double fahrenheit = CtoF(celsius);
                        printf("digital value: %s  celsius: %f  fahrenheit: %f\n", buffer, celsius, fahrenheit);
                        lseek(fd, 0, 0);
                }
                sleep(1);
        }

        close(fd);
}

This program will loop indefinitely. Once a second it will read the/sys/devices/platform/tsc/ain2 which is the ADC (AIN1) interface connected to the TMP36. If it gets back a value the program will then calculate the temperature and print it out. It’s all pretty straight forward. A couple of notable things:

• The string returned by the read is not null terminated.
• After a read the file descriptor needs to be rewound to the start of the file using lseek.

Here’s some sample output.


digital value: 1658 celsius: 22.861328 fahrenheit: 73.150391
digital value: 1655 celsius: 22.729492 fahrenheit: 72.913086
digital value: 1661 celsius: 22.993164 fahrenheit: 73.387695
digital value: 1667 celsius: 23.256836 fahrenheit: 73.862305
digital value: 1657 celsius: 22.817383 fahrenheit: 73.071289
digital value: 1657 celsius: 22.817383 fahrenheit: 73.071289
digital value: 1663 celsius: 23.081055 fahrenheit: 73.545898
digital value: 1661 celsius: 22.993164 fahrenheit: 73.387695
digital value: 1657 celsius: 22.817383 fahrenheit: 73.071289