e017005.PDF

Low-Cost Temperature Measurement

with a Microcontroller

005

It frequently happens that you have an application in which

you want to measure the temperature of a circuit or the out-

side world. This can be easily achieved using additional

components in the form of ICs, or by using a RC network

and a software routine. However, if all the I/O port pins are

already in use, it’s hard to know what to do. A circuit trick

can provide a solution to this dilemma.

As a rule, modern microcontrollers have RC oscillators with

(relatively large) temperature coefficients. Since instruc-

tions are processed at the speed of the RC clock, the exe-

cution time of a software loop varies with the chip temper-

ature. If your program includes a loop that increments a

counter, you will obtain a count that is different for each dif-

ferent chip temperature, and a specific temperature can be

assigned to each counter value. Of course, it is necessary to

have a highly stable time reference, which may be pro-

vided by the 50-Hz mains frequency (for example) or a sec-

ond (crystal) oscilla-

tor network con-

nected to the micro-

controller. Such a

second oscillator

circuit is used with

low-power micro-

controllers to oper-

ate them at very low

clock rates, such as

32 kHz, in order to

save power. The RC

oscillator is then

only put into play as

needed to meet the

demands placed on the software.

µC

RC oscillator

internal capacitance

crystal oscillator

014087 - 11

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