ESP8266 Community Forum

Posted: **Mon Jan 01, 2018 2:07 pm**

I have a project with Wemos mini. I have used i2c to hook up a lcd and a rtc on that bus line. D1 and D2 pins. I like to have more digital pins with a sn74hc595 on the same bus line. What I have been reading the 595 will require 3 digital pins. Can the same i2c bus be used with just adding an extra digital pin?

Posted: **Mon Jan 01, 2018 3:24 pm**

It is difficult to have different communications busses on the same lines. It is easy to cause unexpected problems. Since you are already using I2C I strongly suggest a I2C port expander chip.

The MCP23017 is a good choice since it can provide a strong high output as well as sinking current. 25mA sink/source capability per I/O

https://www.aliexpress.com/item/MCP2301 ... 91000.html

The PCF8575 is often used for driving character LCD modules. It is a simpler device and it can not source much current. (weak high) It can sinck current better than source.

https://www.aliexpress.com/item/PCF8575 ... 13090.html

Posted: **Mon Jan 01, 2018 3:30 pm**

rudy wrote:It is difficult to have different communications busses on the same lines. It is easy to cause unexpected problems. Since you are already using I2C I strongly suggest a I2C port expander chip.

The MCP23017 is a good choice since it can provide a strong high output as well as sinking current. 25mA sink/source capability per I/O

https://www.aliexpress.com/item/MCP2301 ... 91000.html

The PCF8575 is often used for driving character LCD modules. It is a simpler device and it can not source much current. (weak high) It can sinck current better than source.

Thank You

https://www.aliexpress.com/item/PCF8575 ... 13090.html

Posted: **Mon Jan 01, 2018 5:45 pm**

Could you have a mcp23s17 and mcp3008 on spi bus. I have the mcp3008 on the spi can I add the mcp23s17? Or wait until I get the PCF8575 to use on the I2c bus.

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