Subj : Re: RP400 40-pin connector
To   : John Larkin
From : Theo
Date : Mon Aug 12 2024 17:39:35

XPost: sci.electronics.design

In comp.sys.raspberry-pi John Larkin <jjlarkin@highlandtechnology.com> wrote:
> On Sun, 11 Aug 2024 16:22:04 -0400, "Edward Rawde"
> <invalid@invalid.invalid> wrote:
>
> >"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message news:d21ibjdvt6odqium3ado62ob1e7sa6n9hc@4ax.com...
> >>I want to use an Raspberry Pi 400 (the keyboard thing) as the
> >> dev/debug system for an RP2040 based product.
> >>
> >> https://www.amazon.com/Raspberry-Computer-Keyboard-Layout-Kabel/dp/B08QCQVWH2
> >>
> >> It has a 40-pin connector on the back. Various sources say that pins 1
> >
> >This connector?
> >https://www.pi4j.com/1.3/images/pi4j-rpi-400-pinout.png
>
> Yes.

Be aware that WiringPi pin numbers are only useful if you want to use that
library (or similar which use the same mapping).  The real BCM pin numbers
are likely more useful.

I normally go by:
https://pinout.xyz/
which gives all the optional features if you click on a pin.  It also shows
the differences between boards (eg pin 13 is different on Pi 1 rev 1).

It seems the 400 is identical to the 4, since it's the same SoC in a
different case.

> >> 3 and 5 are either GPIO ports 8 9 and 7 or maybe 2 3 and 4.
> >>
> >> Sometimes the pins are labeled WPI and BCM. Wot's that?

WiringPi pin numbers and Broadcom pin numbers probably.

> >> https://www.amazon.com/Coolwell-Waveshare-Raspberry-Adapter-Expansion/dp/B08RZCR7S8
> >
> >If that's just a connector then continuity check it to the above drawing of the connector on the Pi.
>
> The real question is whether pin 3 is GPIO8 or GPIO2, and which two
> pins are the SW debug.

Pin 3:

"GPIO 2 (I2C Data)
Alt0	Alt1	Alt2	Alt3	Alt4	Alt5
I2C1 SDA	SMI SA3	DPI VSYNC	AVEOUT VSYNC	AVEIN VSYNC	

    Physical/Board pin 3
    GPIO/BCM pin 2
    Wiring Pi pin 8
    GPIO/BCM pin 0 on Rev 1 ( very early ) Pi

SDA (I2C1 Data) is one of the i2c pins on the Pi, learn more about i2c.

SDA includes a fixed, 1.8 kΩ pull-up to 3.3v, which means this pin is not
suitable for use as a general purpose IO where no pull-up resistor is
desired."
https://pinout.xyz/pinout/pin3_gpio2/

Pis 0-4 (inc 400) don't have SWD debug, they have JTAG (pins 13, 15, 16,
18).  I think you need something in config.txt to enable ARM-side JTAG.

(the Pi 5 does use SWD, on a separate header).

> On the Pi4, pin 3 is GPIO2. It seems like the pins are renamed on the
> Pi 400, where pin 3 is GPIO8. Why would they do that?

That's the WiringPi confusion.

(I think in the early Pi 1 days the pin numbering - like other parts of the
hardware design - was quite confused, and WiringPi came up with their
numbers to make sense of it.  I think they're mostly just an additional
headache now)

Theo

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