Bi-directional data line, acknowledge problem due to changin

[Nigel]

Hello,
I am using the PIC 16F877 to communicate with a humidity sensor SHT15. The sensor uses a bi-directional data line. First you send the sensor a set of code to start it and tell it to measure humidity when the sensor is ready to send the measurement back, it acknowledges being ready by pulling the data line low. The problem is to register this the PIC needs to be read the change as an I/P, to do this I have changed the port from an output to an input using the TRIS statement. But this changes the state on the port from a 1 to a 0, thus causing an error. Has anyone any ideas.
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This message was ported from CCS's old forum
Original Post ID: 12567

[Sherpa Doug]

:=Hello,
:=I am using the PIC 16F877 to communicate with a humidity sensor SHT15. The sensor uses a bi-directional data line. First you send the sensor a set of code to start it and tell it to measure humidity when the sensor is ready to send the measurement back, it acknowledges being ready by pulling the data line low. The problem is to register this the PIC needs to be read the change as an I/P, to do this I have changed the port from an output to an input using the TRIS statement. But this changes the state on the port from a 1 to a 0, thus causing an error. Has anyone any ideas.

Does your sensor actively pull the line high when it is not ready, or does it expect a pull-up resistor? If it uses a resistor, the resistor should keep the line pulled up when the pin switches to an input. The only way the lins should go low is if the PIC or the sensor is pulling it low.

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This message was ported from CCS's old forum
Original Post ID: 12570

[Michael Grant]

Have you got a pull-up resistor on the line? If the responding chip is an open collector, then where is the voltage to come from?

:=Hello,
:=I am using the PIC 16F877 to communicate with a humidity sensor SHT15. The sensor uses a bi-directional data line. First you send the sensor a set of code to start it and tell it to measure humidity when the sensor is ready to send the measurement back, it acknowledges being ready by pulling the data line low. The problem is to register this the PIC needs to be read the change as an I/P, to do this I have changed the port from an output to an input using the TRIS statement. But this changes the state on the port from a 1 to a 0, thus causing an error. Has anyone any ideas.
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This message was ported from CCS's old forum
Original Post ID: 12611

[meiling]

i have acquired several SHT15 sensors and i am using them to monitor temperature gradients inside a room. i am already able to retrieve data and convert them to actual values. the problem i have is i observe fluctuations of almost +/-6degC in the readings. is this normal even though i can not think of anything that might cause this sharp rise? i sample every 15seconds.

here's a sample of several datapoints i have:

counter, LM35, SHT15
...
12,22.070, 33.160
13,22.119, 33.200
14,22.119, 26.960
15,22.070, 25.720
16,22.070, 25.080
17,22.070, 31.000
18,22.070, 25.720
19,22.021, 24.840
20,22.021, 30.720
21,22.021, 31.760

thank you.

[rnielsen]

I've used this sensor quite a bit. It's response time is fairly quick and I haven't noticed it to drift, randomly, in any of my applications. You might want to test it in an area that does not have any air movement. It's possible that the room air is moving, quite a bit, causing the air to fluxuate.

Ronald

[Humberto]

[asmallri]

I use the Sensirion sensors extensively. Based on your results I would be looking for a hardware (missing pullup resistor?) or software driver problem.

Use the crc field to validate the record. If you are getting CRC errors then you have a hardware related problem. If the CRC is valid then your problem is in your driver.
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Regards, Andrew

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