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I2C GPIO expander questions - PCF8574

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KeepItSimpleStupid

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The datasheet is here:

http://www.electro-tech-online.com/custompdfs/2012/08/PCF8574.pdf

In general, can the chip run at Vss = 5V and the I2C bus SDA and SCL be pulled up to 3.3 V

The processor runs at 3.3 V with 5V tolerant inputs (with a series resistor for other inputs) processor.

What about the interrupt line. I should be able to do one of two things, right:
1) Series resistor
2) 5V to 3.3 logic translator and a 3.3 to 5V translator

My initial playing will be without this chip using two interrupts and some I/O ports directly using level translators. The processor will be this one: http://www.coridiumcorp.com/SuperPRO.php Which is an ARM Cortex M3 @ 100 MHz - LPC1756

Just getting into this new fangled stuff.
 

languer

Active Member
It's close, but it probably could (for +5V, the minimum input for logic levels is +3.6V -> 0.7Vcc). You need to run the I2C chip at +5V? Can you run it at +3.3V as well? That eliminates any sort to level converters.
 

KeepItSimpleStupid

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Input SCL; input/output SDA
VIL LOW level input voltage -0.5 - +0.3VDD V
VIH HIGH level input voltage 0.7VDD - VDD + 0.5 V

I/Os
VIL LOW level input voltage -0.5 - +0.3VDD V
VIH HIGH level input voltage 0.7VDD - VDD + 0.5 V

That makes the levels <.99 low and >2.31 high for 3.3 V supply and <1.5 V and >3.5 V for 5V supply.

TTL is about >2 V for high and < 0.8 V for low.

It does mean that inputs have to be open collector from the 5V side. And 400 uA max input doesn't make these 5V tolerant parts.

OOps!
 
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