rPi-MFC GPIO Inputs CN2/CN3/CN4

CaptainMurdoch

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Are the GPIO inputs CN2/CN3/CN4 on the rPi-MFC wired through a voltage shifter or are they directly wired to the GPIO pins?

We had someone ask about using DPIPixels with the rPi-MFC so that they could have onboard audio back. The main issue there is that the rPi-MFC has an onboard EEPROM which may or may not be programmed in some cases. Using DPIPixels would require a new config added to the onboard EEPROM if it was programmed.

While I was looking at creating a string config for DPIPixels for the rPi-MFC, I noticed that the cape has 3 sets of headers (CN2/CN3/CN4) which are described as being for GPIO inputs. These pins map to 3 of the 24 available DPI pins which means that they could actually also be used for WS281x pixel data if they are hooked up directly.

Theoretically, you could also make a powered or non-powered daughterboard which would plug into the LED Panel connector on the cape and provide WS281x data for 10 more pixel strings using the data and control lines normally used for the LED panels. If the daughterboard was non-powered, you would have to inject power separately, but the voltage shifter is already there for the data and a DPIPixels config could be made which would allow pixel data on those pins.

Alan, if you or anyone wants to discuss options and possible string config files for this cape, I am willing to help get configs generated depending on what makes sense for users.
 

Skymaster

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Hi Captain,

Some detail here - https://www.hansonelectronics.com.au/wp-content/uploads/2020/01/rPi-mfc-user-manual-prelim2.pdf
I'd hazard a guess and say there are no level shifters on them, certainly not for output purposes, as they are detailed for use as "inputs for buttons" (page 9).
Likewise there are the three headers for the Up/Down/Enter buttons.

A very basic EEPROM exists linked from the product page - http://www.hansonelectronics.com.au/product/rpi-mfc/ - however this is only configuring the button headers, RTC and OLED.
It doesn't seem to have anything specific for the string outputs (Even in the non-DPIPixels mode).


(Happy to be corrected on the above if I've misinterpreted it :D)
 

CaptainMurdoch

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I have that EEPROM, have extracted it, and looked at the files, etc. and have looked through the manual, that's how I confirmed which GPIO pins the headers were wired to and that they would be compatible with the DPI output. The main question is are they hardwired through a voltage shifter as an input or are they directly wired to the GPIO pins and could also be used as a 3.3V output. Adding a single null pixel would allow you to use those as 3 WS281x string outputs and run the single-string cape as a 4-string cape using DPIPixels. Or if a daughterboard was made and used with the LED panels connector as I noted, that could give you a 14-string cape using DPIPixels, 11 of them with the data line at 5.0v and 3 at 3.3v.
 

AAH

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The 3 inputs have 1k resistors in series with them to direct to the inputs on the Pi. Using them direct you wouldn't be able to run pixels I wouldn't think as even putting the pixel right at the board would probably be too much resistance to run the pixel properly. I do suspect that putting 1 of my null pixel buffers or similar would allow the inputs to be used as outputs successfully.
 

CaptainMurdoch

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Jan 21, 2013
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OK, thanks for the info, so that rules those GPIO headers out.

Well, we could make a new EEPROM .bin that included an option for using DPIPixels for the single WS281x output at least. I'm tossing around some ideas to make upgrading the EEPROM image easier for cases like this and for other vendor EEPROM updates. At some point, if someone did make a pixel string daughterboard to plug into the LED Panel header, a new config could easily be added to support that. One interesting note is that a daughterboard like that could allow any LED panel cape to support 6 string of pixels using the 6 dedicated RGB data lines available on a LED panel connector.
 
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