FT232R and MAX232 on same TX and RX

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#1
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I have tried to use the ftdi and maxim chip sharing the same TX and RX pins on a ATMEGA644P. It seems they both are able to transmit(uC sends data to computer), but only the MAX232 can receive (computer sends data to uC). I think I might have an idea why this is. The pins of the chips connected to the atmega's TX pin are inputs. That means the uC can drive them high or low no problem. But the pins of the chips connected to the atmega's RX pin are outputs. This means that when one chip tries to drive the atmega's RX pin high or low, it also must drive the other chip's output high or low. The MAX232 can handle this and the FT232R can't? Just an idea. Any suggestions?

Thanks,
Andy

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You have guessed correctly.

You can insert a 1k resistor between the AVR RX pin and each device.

Then ideally you need to disable the unused device with its CS or CE pin. But in practice you may just about manage to receive from either device.

A better solution is to insert a gate between the two devices and select with an AVR GPIO pin.

Moral of the story is: Use FTDI on USART1 and MAX232 on USART0

David.

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Thanks, I'll remember that for the next PCB run. I hacked in some 1K resistors like you said and it works now.

Thanks,
Andy

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In apps like this, we use a mux. It seems we usually end up with a 4x2 part even when only 2x2 is needed. ;) In general either a digital or analog mux will work.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.