16-bit I/O expander for I2C-bus "PCF8575" Problem? Simple Project!

Thread Starter

Elazar

Joined Oct 29, 2019
35
Hi,

I am experimenting with the PCF8575 IO expander chip, Datasheet

PROBLEM DESCRIPTION:
After setting an IO as an input, the pin will be pulled to the ground, and I don't see a way to configure it to be high impedance while as an input?

However, if I connect the input pin to VCC it will be read as high BUT it might short the circuit (so I need to put a resistor)

Question:
is it possible to set the IO as a high impedance input?

Am I missing something?

See logic diagram:
1621343777067.png
 

Papabravo

Joined Feb 24, 2006
16,104
Same as the Intel MCU ports.. Bazaar!! The MCP23017 from Microchip have fully configurable IO's
I think you meant bizarre, and I agree.
A Bazaar is an open air market where you can buy things from street merchants. Homophones are wonderful for satire - don't you agree?
In the same category as the malaprop. I love it when I see a dancer, dancing a flamingo.
 

Thread Starter

Elazar

Joined Oct 29, 2019
35
So the answer is that it can't be configured as high impedance?
Only connected to VCC or GND?

It doesn't make any sense?
 

andrewmm

Joined Feb 25, 2011
1,464
So the answer is that it can't be configured as high impedance?
Only connected to VCC or GND?

It doesn't make any sense?

No

it says , set the output to '1',
then do a read,
It says this disables the low impedance pull up
"after this short time, the output is in 3 state mode."
 

Ian Rogers

Joined Dec 12, 2012
888
I think you meant bizarre, and I agree.
A Bazaar is an open air market where you can buy things from street merchants. Homophones are wonderful for satire - don't you agree?
In the same category as the malaprop. I love it when I see a dancer, dancing a flamingo.
Yep... I knew it was wrong when I typed it.... But!!! who cares.... either or... the port pins on an Intel are not the best...

Don't get me wrong! I've used these for some time and only had one or two issues...( Me mainly ) but I much prefer microchips and AVR's IO system..
 

Papabravo

Joined Feb 24, 2006
16,104
The high impedance pullup and the absence of a pulldown means that an external source can drag the pin around between its limits without causing any heartburn. It is exactly the same thing as dragging the pullup on an open collector gate around.
 

Papabravo

Joined Feb 24, 2006
16,104
Yep... I knew it was wrong when I typed it.... But!!! who cares.... either or... the port pins on an Intel are not the best...

Don't get me wrong! I've used these for some time and only had one or two issues...( Me mainly ) but I much prefer microchips and AVR's IO system..
It was literally the dawn of electronic time when they came up with the concept of a bi-directional I/O port pin (ca. 1974-75). I wouldn't be too hard on them. Those that came later had the advantage of seeing the shortcomings which to my mind are pretty trivial considering the advantages it gave to the single-chip microcontroller.
 
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