Rpi 4 Pin 29 default to High pull?

Question

I am fairly new to Raspberry PI GPIO pins. I have some simple code which sets Pin 29 (BCM 5 in this picture) as an IN (READ) Pin and then reads from it after boot and on raspberry pi 3 it has always been Low however on the raspberry pi 4 it seems to be High.

I am trying to understand why this is the case.

I was not able to find any specification on why that is. My understanding is that the raspberry pi 4 uses BCM2838 however the only spec sheet I can find is this one for the BCM2835 (https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf)

On page 102 there is a table with alternate functions showing the default pull values and for Pin 29 is has a dash (-) and some comment at the bottom about white values not to be used but I am not certain whether this is relevant

The Go code below reproduces the issue by performing a simple READ on the pin (We use the latest periph library with support for rpi 4

package main

import (
    "fmt"
    "log"
    "time"

    "periph.io/x/periph/conn/gpio"
    "periph.io/x/periph/host"
    "periph.io/x/periph/host/rpi"
)

var (
    readPin  = rpi.P1_29
)

func main() {
    fmt.Println("Initializing periph host...")
    if _, err := host.Init(); err != nil {
        log.Panicf("failed to initialize periph host: %v", err)
    }

    if err := readPin.In(gpio.PullDown, gpio.NoEdge); err != nil {
        log.Panicf("failed to set read pin to In(): %v", err)
    }

    ticker := time.NewTicker(2* time.Second)
    want := gpio.Low

    for {
        select {
        case <-ticker.C:
            time.Sleep(time.Millisecond)
            actual := readPin.Read()
            if actual != want {
                log.Printf("ERROR: Actual: %t != Wanted: %t", actual, want)
                continue
            }
            log.Println("EVERYTHING WAS FINE")
        }
    }
}

Any chance somebody can shine a light on the situation? Is there some undocumented behaviour to Pin 29 I am missing causing this? Is this expected behaviour (and if so an explanation would be nice)

score 2 · Answer 1 · answered Jan 15 '20 at 11:18

On all Pis with the 40 pin expansion header pin 29 is connected to GPIO 5.

GPIO 5 defaults to a pull-up at power up.

The pulls are quite weak (about 50k ohms) so they can easily be overridden by an external voltage.

The table you linked shows GPIO 5 as defaulting to pull-up.

GPIO 28/29 are not brought out to the 40-pin expansion header.

Rpi 4 Pin 29 default to High pull?

1 Answers1