OpenSprinkler Forums Hardware Questions OpenSprinkler ESP8266 Datasheet and Logic Levels questions

  • This topic has 3 replies, 3 voices, and was last updated 2 weeks ago by Ray.
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  • #71545

    DaveTheWhiteAsian
    Participant

    Hey Everybody,

    I’ve been searching the internet for a Datasheet for the ESP8266 but have been unsuccessful at this point. If anybody has this could you shoot it my way?

    … we’re having trouble with the interface between the master and slave modules. Looking through the schematics and probing voltages, it appears that the slave module’s I/O expander operates on +5V, while the master module generates 3.3V logic signals. It looks like these 3v3-logic signals are at the cusp of being interpreted by the PCA9555 I/O expander on the slave module. Has anyone else found this?

    #71567

    Ray
    Keymaster

    ESP8266’s technical documents are all listed here:
    https://www.espressif.com/en/products/socs/esp8266

    PCA9555 is a I2C device, for I2C device, because the data lines are pulled high by the microcontroller, and the slave devices only pull these data lines to ground, it’s ok to have a 3.3V microcontroller interfacing with a PC9555 powered by 5V. 3.3V is within the high logic range of PCA9555, so this works fine.

    #71650

    PeterTheRed
    Participant

    Hi Ray, Thanks for the response. My name is Peter, and I’m working with Dave on this issue.

    Looking at the datasheet for the PCA9555, I see that VIH, input HIGH voltage for SDA/SCL pins > 0.7 * Vdd. (see attached screen capture)

    For Vdd = 5.0V, 0.7*Vdd = 3.5V, which is higher than the 3.3V logic signals generated by the MCU.

    I’m working on adding a FET based level shifter to this ckt and some local pull-ups to 5V on the slave-side to see if that doesn’t resolve things.

    Thanks,
    Peter

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    #71653

    Ray
    Keymaster

    Yes, I am aware of the datasheet, but we’ve never had any problem using 3.3V ESP8266 to drive 5V PCA9555. If you believe it’s a problem you can simply add a diode in series between 5V and PCA9555’s VCC pin, so that will drop 0.7V and supply 4.3V to PCA9555. This way 3.3V/4.3V > 75%.

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OpenSprinkler Forums Hardware Questions OpenSprinkler ESP8266 Datasheet and Logic Levels questions