Boot Issue with Raspberry Pi 3B+

[dhardingham]

With the ReproPrinter attached to a Raspberry Pi 3B+, the Raspberry Pi fails to boot. The power light (red) on the Raspberry Pi is on, but the activity light (green) doesn't do anything. It should flash, to show I/O activity on the MicroSD card.

The rainbow screen isn't displayed, the screen is blank.

Using a multimeter, I have discovered that the 3v GPIO pins have been pulled down to 1.84v. This is what stops the Raspberry Pi from booting, since the GPU needs a 3v power rail.

There is nothing attached to the RetroPrinter Centronics port.

If I boot the Raspberry Pi 3B+ without the RetroPrinter attached, everything works as expected. I can attach the RetroPrinter after the Raspberry Pi has booted, and everything still works correctly. Rebooting the Raspberry Pi 3B+ with the RetroPrinter attached is no problem, just the initial boot.

[RWAP]

Certainly one other person has successfully used the Retro-Printer with the Raspberry Pi 3 Model B+

I wonder if it might be a faulty Retro-Printer module? Have you tried using the actual software image we provide just to eliminate a potential problem with the HAT itself?

[RWAP]

I was discussing this with the hardware designer (Sascha) - when we realised that you were using your own software image.

You probably need to remove the Retro-Printer Module and disable the serial port pins, as we use those as extra input / output:
https://www.cube-controls.com/2015/11/0 ... -raspbian/

[dhardingham]

After trying several different images, Rapberry Pis and microSD cards, I'm convinced that the Pi 3B+ fails to boot because of a hardware issue.

I've always been convinced that this was a hardware issue, so I decided to build a test rig using an Adafruit T-Cobler Plus and an Adafruit Cobler Plus. I now have full access to all of the GPIO pins, to measure voltages and current flows.

With the RetroPrinter attached to the Raspberry Pi (via the test rig), I turned the power on and the 3B+ failed to boot - just as I expected. The red LED was on, with no activity on the green LED. Unfortunately, GPIO Pin 1 was no longer delivering 3.3v, it was only delivering 1.8v. I assumed that the RetroPrinter was drawing too much current from pin 1, causing a voltage drop. The next step was to measure the current flowing on pin 1.

To my surprise, the current flowing out of pin 1 was -0.028mA!!! Yes, there was a small amount of current flowing the wrong way! This would also explain why the Pi 3B+ failed to boot, since a negative current means there's a negative voltage (reducing the actual voltage on pin 1). Time to turn the power off and prevent the current flowing the wrong way.

I connected a 1N4002 diode between GPIO pin 1 on the Pi 3B+ and GPIO pin 1 on the RetroPrinter. I turned the power on and the Pi booted correctly. In fact everything worked correctly. With my BBC Micro connected to the RetroPrinter, I could print directly from the BBC Micro to my Canon Pixma MG7550 network printer.

I don't know why the RetroPrinter generates a small negative current on GPIO pin 1, but I can understand why this causes a problem with the Pi 3B+. The 3.3v power supply on the Pi 3B+ is very different to earlier power supply designs.

Now all I need to do is make a short 40-pin cable that includes a diode in the pin 1 connection.

[RWAP]

That is interesting - I know others are successfully using the Raspberry Pi 3B+ - perhaps you could confirm the version number of the Raspberry Pi and we could compare it with ones which are known to work!

[RWAP]

There is also an issue with using one of the existing Retro-Printer SD images on the Pi 3B+ - we hope to have a new image to download soon

[RWAP]

We think that the power on pin 1 of the GPIO may well be due to the factory error when soldering the centronics port connector, but will add a diode to pin 1 for future revisions of the PCB.

Interestingly a user with PI 3 Model B+ V.1.2 2015 did not experience this issue.

[dhardingham]

Sounds like someone is getting a little confused with Raspberry Pi model names and versions. The Raspberry Pi 3B+ was released on March 14th 2018 (Pi day). The distinctive features of the 3B+ are the metal heatsinks covering the CPU and the Wifi chip. The version number for the 3B+ is V1.3. Anything with an earlier date, or a lower version number, must be an earlier Raspberry Pi model B. As always, there's a Wiki full of information that helps with model names - https://en.wikipedia.org/wiki/Raspberry_Pi

[RWAP]

Yes, I thought that too. I thought the 3B+ was a bit more modern and the 2018 release date would fit in with the datestamp of 2017 on the PCB.

Did you manage to attach the diode? Any pictures?

[dhardingham]

So here are the pictures of how I attached a small diode and a piece of wire between GPIO pin 1 on the bottom of the Raspberry Pi 3B+ and GPIO pin 1 on the top of the RetroPrinter. Note that I have cut into the connector on the bottom of the RetroPrinter, to break the pin 1 connection. Everything fits neatly inside the RetroPrinter case.