PCF8574A: PCF8574ADWR does not transfer data to output

Vladimir Chromy

Part Number: PCF8574A
Other Parts Discussed in Thread: TCA9534A, TCA9534, , PCF8574

Tool/software:

Hello, I am using PCF8574ADWR connected according to the attached diagram. After sending data to the output, there is no data on the outputs, despite the ACK signal. I am attaching the output from the logic analyzer. I have already used 6 pcs from one tape and they all behave the same. When I use the PCF8574N and change the address to the appropriate circuit, everything works correctly.

Can you advise me?

BR and thank you.

Zosik_Sch.pdf

7 months ago

0 Clemens Ladisch 7 months ago

Guru 321960 points

For a high output, the PCF8547A's output current can be as low as 30 µA. The ULN2803AFWG wants up to 1350 µA at its input.

You can try to add 820 Ω pull-up resistors at the outputs of the PCF. But it would be a better idea to use an I/O expander with stronger outputs like the TCA9534A and to adjust the software for it.

0 Joshua Salinas 7 months ago

TI__Mastermind 34610 points

Hello Vladimir,

Our apps team is current out today celebrating Labor Day (US Holiday). Allows us two days to respond back. Thank you for your understanding.

Regards,

Josh

0 Vladimir Chromy 7 months ago in reply to Clemens Ladisch

Prodigy 10 points

Thank you for your reply.

I ordered a TCA9534 IC and will write the result after trying the device.

I think this is the right direction and thank you once again.

0 Tyler Townsend 7 months ago

TI__Guru 59108 points

Hi Vladimir,

I see Clemens' point about ULN2803AFWG needing up to 1350 uA at its input defined by the following spec in the datasheet:

However, I don't understand why PCF8574ADWR does not work, but PCF8574N does. The "N" denotes PDIP packaging. The "A" version should have a different I2C address. Other than address and package differences, PCF8574 and PCF8574A should have the same output driving structure. What are the differences between the PCF8574N and PCF8574A circuits?

Regards,

Tyler

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PCF8574A: PCF8574ADWR does not transfer data to output