Establish communication with OneWire probe DS18B20 using IO of an IO expanderPCA9574

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1

Hi all,

In our STM32L082 based board we use PCA9574, as we do not have enough pins in the micro-controller.
We have to use one of the IO pin of PCA9574 to establish communication with One Wire probe DS18B20.

I am facing challenges in establishing communication, as the default Arduino based One Wire library will not be helpful and I have developed similar library to use PCA9574 IO pin.
But that does not help in establishing communication.

Can anyone help me with this issue.

Thanks in advance.
Dhinesh

2

Yes, of course we can help:

Use an STM i/o pin to communicate with the DS18B20 sensor and move some less critical function to the pca8574 instead.

2 Likes
3

Thank you @PaulRB . Yes your suggestion is right.

Unfortunately running out of I/O pins in STM for OneWire data, and looking for the possibilities of using I/O pin of IO expander.

4

Are you aware that OneWire is a bus? You can connect many sensors to the same pin.

5

Yes, we can connect number of OneWire device in a same pin as we have different address for each device. Trying to understand how it will be a challenge for using OneWire on an IO expander controlled via I2C ?
Because I were able to configure/control IO pin in the expander by writing a APIs (pinMode, digitalWrite and digitalRead) which made it easier for using in OneWire implementation.

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would like to let you know for our application we are planned only one OneWire sensor in an IO pin.

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You would need to write a new OneWire library, or change the existing OneWire library. This may be difficult even for an expert. It also may be difficult to achieve the correct signal timing that the OneWire protocol requires when using an external i2c i/o adapter chip.

I do not know for certain if this will work, but maybe you can consider a multiplexer like 74hc4067.

1 Like
8

Please list what the pins are used for on the STM and what they are used for on the expander

9

Yes you are right, it is challenging as OneWire is time sensitive, in this case, the timing for I2C operations should also be considered, and that cannot fit the time frame of OneWire protocol.

10

In that case it may be better not to use the one-wire facility. Using a DS18B20 SHEEPDOG

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Are you saying only one sensor for the application or 1 pin each for more than one sensor. If it is the latter then you have a bad design. When designing you need to consider hardware, software and the interaction between them.

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instead of pin from IO expander, planned to use pin from Controller

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as @PaulRB told you in the very first answer...

Please mark his post #2 as a solution.

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