Using universal USB LCD backpack as FT232 breakout

USB and serial backpack PIC_LCD_backpack for HD44780 character LCD screens.

Using universal USB LCD backpack as FT232 breakout

Postby Grapsus » Sat Nov 17, 2012 9:20 am

Hi there !

I noticed that the documentation for the FT232 backpack says, it can be used as FT232 breakout :
http://dangerousprototypes.com/docs/USB ... D_backpack
The problem is that RXD input is driven by a shift register output, and there seems to be no way to disable it.
Do I have to desolder the shift registers to use the PCB as FT232 breakout, or is there a smarter way to do it ?

Thank you in advance
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Re: Using universal USB LCD backpack as FT232 breakout

Postby matseng » Sat Nov 17, 2012 10:14 am

Two ideas...

1) Just cut the trace from the second shift register leading into the FT232. There is no compelling reason for it to be there - why would the computer need to receive the data that it just sent previously?

2) Make the FT232 set its DSR output a high level. This will disable the outputs of the shiftregisters and they will be in high impedance mode so they can't affect the RXD pin of the FT232.
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Re: Using universal USB LCD backpack as FT232 breakout

Postby Grapsus » Sat Nov 17, 2012 12:09 pm

You're right, those shift registers are output only, so there no use in reading data back.

I thought about your 2) solution, but sadly DSR disables only parallel outputs of the shift registers, and RXD is driven by the "serial output" pin, which doesn't have a high impedence mode. Moreover I'd like to be able to use the DSR signal as I wish.

So I think I will go with your first solution and cut the serial output trace.

Thank you !
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Re: Using universal USB LCD backpack as FT232 breakout

Postby matseng » Sat Nov 17, 2012 9:47 pm

Grapsus wrote:I thought about your 2) solution, but sadly DSR disables only parallel outputs of the shift registers, and RXD is driven by the "serial output" pin, which doesn't have a high impedence mode. !

That's true, and it makes sense that the SerialOut-pin would not go into HiZ because that would leave the SerialIn-pin on the next unit in a daisy chain with an undefined level.....
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