I2C PCF8574, PCF8575 I/O expander I can't drive a pin to Output & High - Electronic Circuits Projects Diagrams Free

blueroomelectronics · 6th July 2009, 07:25 PM

I've got a little I2C rig setup and I can communicate with an PCF8575
I can drive a pin LOW but cannot pull it high.
eg:
0x40,0xAA,0xAA turns every other pin low but nothing goes high.
Anyone else using this type of I/O expander.

**Nigel Goodwin** · 7th July 2009, 10:37 AM

Aren't they really strange? - pins are IN and OUT at the same time, and I 'think' that outputs are open-collector, so require pull-up resistors to work.

Chippie · 7th July 2009, 11:04 AM

According to the datatsheet, they are active low open drain outputs..

http://www.nxp.com/acrobat_download/...es/AN469_2.pdf

blueroomelectronics · 7th July 2009, 12:31 PM

What's the 100uA pullup for? Those pins never show high...

Chippie · 7th July 2009, 12:37 PM

Is this what you are referring to?

4. Question: In the data sheet of PCF8575 it is mentioned that the Port output is HIGH
after power-on. In the data sheet of PCF8575C it is mentioned that the Port output will
be in 3-state condition after power-on. We found that there is no difference in the block
diagram and the simplified schematic diagram of each I/O. What is the condition of the
outputs after power-on?
Answer: There is in fact a small difference in the schematic diagrams for PCF8575
and PCF8575C. The PCF8575C lacks the weak pull-up in the form of the 100 μA
current source in the output I/Os. This is the reason that PCF8575's I/Os remain HIGH
after power-on and after being written HIGH. Without this current source, PCF8575C's
I/Os will be in high-impedance state after the initial strong pull-up transistor is OFF (it
is ON during the acknowledge phase).

blueroomelectronics · 7th July 2009, 12:42 PM

Thanks Chippie, I'll see if I can source some with the pull ups.

Chippie · 7th July 2009, 12:45 PM

no probs m8ty...Happy hunting...

blueroomelectronics · 7th July 2009, 03:03 PM

Aye, well I haven't found any other revisions aside from C and D. Now I've got to check for pullups on NXT or TI rev D.

edit:
The PCA9555 is pin compatible and appears to have the resistors! Thanks Chippie.

Chippie · 7th July 2009, 03:43 PM

Quote:

Originally Posted by blueroomelectronics

Aye, well I haven't found any other revisions aside from C and D. Now I've got to check for pullups on NXT or TI rev D.

edit:
The PCA9555 is pin compatible and appears to have the resistors! Thanks Chippie.

Ahh that's good..

I do have a use then?......

blueroomelectronics · 7th July 2009, 04:01 PM

Quote:

Originally Posted by Chippie

Ahh that's good..

I do have a use then?......

But alas, I'm afraid we'll have to sell you for scientific experiments.

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6th July 2009, 07:25 PM	#1
blueroomelectronics	I2C PCF8574, PCF8575 I/O expander I can't drive a pin to Output & High I've got a little I2C rig setup and I can communicate with an PCF8575 I can drive a pin LOW but cannot pull it high. eg: 0x40,0xAA,0xAA turns every other pin low but nothing goes high. Anyone else using this type of I/O expander. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

7th July 2009, 10:37 AM	#2
Nigel Goodwin	Aren't they really strange? - pins are IN and OUT at the same time, and I 'think' that outputs are open-collector, so require pull-up resistors to work. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

7th July 2009, 11:04 AM	#3
Chippie	According to the datatsheet, they are active low open drain outputs.. http://www.nxp.com/acrobat_download/...es/AN469_2.pdf __________________ Eccentric millionaire financed by 'er indoors

7th July 2009, 12:31 PM	#4
blueroomelectronics	What's the 100uA pullup for? Those pins never show high... __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

7th July 2009, 12:37 PM	#5
Chippie	Is this what you are referring to? 4. Question: In the data sheet of PCF8575 it is mentioned that the Port output is HIGH after power-on. In the data sheet of PCF8575C it is mentioned that the Port output will be in 3-state condition after power-on. We found that there is no difference in the block diagram and the simplified schematic diagram of each I/O. What is the condition of the outputs after power-on? Answer: There is in fact a small difference in the schematic diagrams for PCF8575 and PCF8575C. The PCF8575C lacks the weak pull-up in the form of the 100 μA current source in the output I/Os. This is the reason that PCF8575's I/Os remain HIGH after power-on and after being written HIGH. Without this current source, PCF8575C's I/Os will be in high-impedance state after the initial strong pull-up transistor is OFF (it is ON during the acknowledge phase). __________________ Eccentric millionaire financed by 'er indoors

7th July 2009, 12:42 PM	#6
blueroomelectronics	Thanks Chippie, I'll see if I can source some with the pull ups. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

7th July 2009, 12:45 PM	#7
Chippie	no probs m8ty...Happy hunting... __________________ Eccentric millionaire financed by 'er indoors

7th July 2009, 03:03 PM	#8
blueroomelectronics	Aye, well I haven't found any other revisions aside from C and D. Now I've got to check for pullups on NXT or TI rev D. edit: The PCA9555 is pin compatible and appears to have the resistors! Thanks Chippie. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/ Last edited by blueroomelectronics; 7th July 2009 at 03:32 PM.