piclist 1998\11\16\165130a >
Thread: Question
picon face BY : Peter L. Peres email (remove spam text)

On Mon, 23 Nov 1998, Martin wrote:

> Hello, I need to know the cheapest way to measure the temperature of a
> processor and the temperature of some water, and have the output be in
> serial format. I have no experience programming PICs myself, but I do
> have experience with all types of hardware. Is there a way to do this
> without having to use a PIC ?

For the water, many people will suggest a certain Dallas part.

For the processor however, I think that I've come up with a truly evil way
to read the die temperature without opening a chip. Here is the idea:

On nearly all micros there are pins that remain unused in a certain
applications. Most of these have an intrinsic protection network built in
which includes a protection diode towards Vdd. There is also a higher
voltage source available (the unregulated power supply).

The idea is to supply a constant current into an unused input pin and bias
the protection diode in reverse, and buffer the voltage difference Vinput
- Vdd, and divide it (by 2 ?) to get it into the range of an on-chip A/D.
Thus a PIC with A/D should be able to measure its own die temperature
while working ;).

A Si diode has a temp. co. of -2.2... mV/K but it needs to be calibrated
for the resp. range. A gain of x 10 after dividing by 2, using a suitable
DC reference should allow a temperature range of 100 degrees C on 8 bits.
This diode should be very closely coupled to the die. It is also very
closely coupled to any voltage drop on the Vdd grid inside the chip which
would require some software measures to be taken while sampling the temp.
dep. voltage (2.2 mV ~= 10 mA on 0.2 ohms...).

this needs to be elaborated, but how does it strike you ? ;)



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