Simple Buffer Board, ideas.


6 posts
by domesday » Fri Jun 22, 2012 1:54 pm
I know there are a lot of people making their own expansion boards of various designs but I am planning on designing a simple one for people who don't need something as fully featured as the Gertboard.

Basically my idea is simple board no bigger than 40mm x 56mm providing buffered I/O for the 8 GPIOs, the ones without special purposes and perhaps some grouped pins for the serial interfaces.

The 8 lines would be 5V input tolerant and a jumper would select between 3.3 an 5V output levels that would provide up-to 50mA so that it could be used with many of those breakout boards etc that are made for Arduino etc.

The first question is that there are two ways of handling the selection of input or output function. It could be done with 8 jumpers so that each line could be configured as in or out, or it could be simplified so that the 8 lines could be configured in two blocks of 4. So using 2 jumpers it would be possible to select 8in/4in+4out/8out, this would make it a bit simpler but slightly less flexible.

The second question is, screw terminals or male/female header ?

Any other suggestions welcome.
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by iggykoopa » Fri Jun 22, 2012 5:44 pm
I was just planning on using one of these http://www.ti.com/lit/ds/symlink/sn74hc244.pdf to buffer the lines, since I have a few on hand. It wouldn't be selectable for input or output, but just using it to play around with stuff shouldn't be a big deal.
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by mahjongg » Fri Jun 22, 2012 6:46 pm
Would be fine, but be sure to power then from 5V if you attempt to drive an RPI input with them.
In any case place a small (1K) resistor in the line between the GPIO and the buffer just in case you try to use the GPIO as output.
If you want to use them as ouput level converters they should also work to "buffer" the GPIO when you power them with 5V.
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by iggykoopa » Fri Jun 22, 2012 7:04 pm
Thanks for the tips, I'm still pretty new with the electrical side of things. I picked those up for an avr dragon, so figured I would grab a couple extra.
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by domesday » Fri Jun 22, 2012 7:08 pm
mahjongg wrote:Would be fine, but be sure to power then from 5V if you attempt to drive an RPI input with them.


Powering them from 5V on a RaspberryPI input would result in a 5V high on the output, the Raspberry Pi can only accept 3.3V, so I'm not sure why you are suggesting this ?
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by mahjongg » Sat Jun 23, 2012 11:40 am
Oops, I meant powering the chip from 3V3, not 5V.
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