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I have read somewhere, but I have not bookmarked it. Maybe it was a post in one of the forum for Rpi that

  • GPIO 0-8 are pull up enabled when the Pi starts up
  • GPIO 9-27 are pulled down enabled when Pi starts up

I tried to validate that in my RPi 4 Model B, I tested GPIO Pins 0 to 8 when the RPi boots up and I connected my multimeter to ground for each GPIO Pins.

I checked that the output from GPIO 0-8 is indeed 3.3 or less, the other Pins are 0V.. however when I checked, GPIO 14 and GPIO 15 which is TXD and RXD use, I am getting a 3.3 v also.

Is what I have read wrong? I can't find a documentation about this.

UPDATE I executed the command suggested and I was able to verify that it GPIO 14 and GPIO 15 was indeed pulled up.

raspi-gpio get
BANK0 (GPIO 0 to 27):
GPIO 0: level=1 fsel=0 func=INPUT pull=UP
GPIO 1: level=1 fsel=0 func=INPUT pull=UP
GPIO 2: level=1 fsel=4 alt=0 func=SDA1 pull=UP
GPIO 3: level=1 fsel=4 alt=0 func=SCL1 pull=UP
GPIO 4: level=1 fsel=0 func=INPUT pull=NONE
GPIO 5: level=1 fsel=0 func=INPUT pull=UP
GPIO 6: level=1 fsel=0 func=INPUT pull=UP
GPIO 7: level=1 fsel=1 func=OUTPUT pull=UP
GPIO 8: level=1 fsel=1 func=OUTPUT pull=UP
GPIO 9: level=0 fsel=4 alt=0 func=SPI0_MISO pull=DOWN
GPIO 10: level=0 fsel=4 alt=0 func=SPI0_MOSI pull=DOWN
GPIO 11: level=0 fsel=4 alt=0 func=SPI0_SCLK pull=DOWN
GPIO 12: level=0 fsel=0 func=INPUT pull=DOWN
GPIO 13: level=0 fsel=0 func=INPUT pull=DOWN
GPIO 14: level=1 fsel=2 alt=5 func=TXD1 pull=NONE
GPIO 15: level=1 fsel=2 alt=5 func=RXD1 pull=UP

In Page 102,of this https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf documentation, it says that GPIO 14 and GPIO 15 should indeed be pulled down. So is there something in my setup that is pulling this up?

Note I just rebooted my Rpi 4 several times.

Mark Estrada
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  • How come there is no document? – tlfong01 Jun 19 '20 at 02:23
  • And I don't know at boot, which pins are output, which are input. I only know that if a pin is input, then it matters whether there is a pull up or pull down. Neither do I know if the pull up or down of an output pin can or cannot be disabled. But I do know know that output pins' pullups cause confusion trouble, say, in I2C circuits. In short, I am confused, and I am not sure if what I learned in Rpi3 applies in Rpi4. – tlfong01 Jun 19 '20 at 04:14
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    I actually am just exploring each of the things that I am reading so I am validating it. I just started fiddling with my RPi this month. I find this interesting though. – Mark Estrada Jun 19 '20 at 04:27
  • Ah, yes, you remind me that I also started exploring things when I first played with my Rpi1 some years back. As I said earlier, I could not find documents to confirm at boot which pins are output, which are input, and which pullup/down are enabled or disabled. It would be nice if you can share your experiment results with a chart, so me and other newbies would no longer weeping for this newbie sorrow in the dark. – tlfong01 Jun 19 '20 at 04:50

1 Answers1

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There is an image which shows default pullup (among other things) http://www.panu.it/raspberry/

NOTE the above is not strictly correct for the Pi4. The Pi4 default pin settings (and most common optional settings) are identical, but the Pi4 has additional settings for some pins.
See Pi4 GPIO

The BCM2711 ARM Peripherals which can be downloaded is the official Pi4 documentation. Section 5.3. Alternative Function Assignments shows possible functions.

The Answer by joan explains and references the official documentation https://raspberrypi.stackexchange.com/a/32643/8697

The command

raspi-gpio get

will show the the ACTUAL state of GPIO (including pullups for Pi4 only). See https://raspberrypi.stackexchange.com/a/113414/8697

PS Using a multimeter is not a reliable method of determining pullup.

Pins 14,15 are configured as serial. TxD is an OUTPUT.

Milliways
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  • I have run the command and I saw indeed that the GPIO Pins 0-8 are pulled high. Also my GPIO Pins 14 and GPIO 15 are pulled up as well. GPIO 14: level=1 fsel=2 alt=5 func=TXD1 pull=NONE GPIO 15: level=1 fsel=2 alt=5 func=RXD1 pull=UP – Mark Estrada Jun 19 '20 at 02:54
  • So in the documentation in here also https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf, Page 102.. it says it should be pulled down. So is there something in my Rpi4 that pulls up GPIO 14 and 15? Thanks for the useful link by the way – Mark Estrada Jun 19 '20 at 02:57
  • Ok thanks noted in that – Mark Estrada Jun 19 '20 at 04:09