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Maximum pins that can be used for i/o in RaspberryPi2 J8 header?

Former Member

Hi,

 

I am planning to use Raspberrypi2 for making a hardware. I am using Rpi2 as the processing part. I need 27 pins to be interfaced. I see that there are 17 gpio pins on the J8 header.

My question is can I use spi and i2c pins as input/output pins? Also the reserved pins can be used for i/o?

 

Regards,

Hidayat

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  • 0

    Any of the pins labeled as GPIO can be used as general purpose I/O however some of the pins will be configured for special purposes by the kernel at boot time (the serial I/O pins are the only two that I am sure of)

    and will have to be reconfigured for general purpose I/O after booting up.

     

    Even with all pins reconfigured I still count only 26 GPIO pins available. It would help to have some idea of what you want to use the I/O for as there are several ways (latches, shift registers, I2C I/O expanders, etc.)

    to get more I/O out of the pins you have.

  • 0 in reply to gdstew

    Gary, I am not 100 % sure but as the special GPIO ports can be configured enabling I2C instead of other features from the kernel in the same way can be set to off (i.e. general purpose I/O only). I suppose this is the reason that depending on the connected hardware these features can be enabled / disabled with the raspi-config command (that changes the kernel setting and in these cases requires reboot)

     

    Enrico

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  • 0 in reply to gdstew

    Gary, I am not 100 % sure but as the special GPIO ports can be configured enabling I2C instead of other features from the kernel in the same way can be set to off (i.e. general purpose I/O only). I suppose this is the reason that depending on the connected hardware these features can be enabled / disabled with the raspi-config command (that changes the kernel setting and in these cases requires reboot)

     

    Enrico

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  • 0 in reply to balearicdynamics

    If you do not enable I2C or SPI using raspi-config then these pins should not configured for these functions by the kernel during boot and they should be usable as GPIO.

    If they have already been enabled then you can disable them using raspi-config and they should be usable after rebooting. This should also take care of any "unexpected"

    pull-up resistors being used (John).

     

    Also, the serial I/O pins (RXD0, TXD0) are normally set up at boot time for use as a serial console terminal for Linux. I'm not sure if removing "console=ttyAMA0,115200"

    from the command line in the file cmdline.txt (copy this file to cmdline.txt.orig first) in the /boot directory will stop this but I believe it will. These settings can also be overridden

    during or after the Linux system initialization (sys v init) but during boot a lot of 115K baud async serial boot text messages will appear on the TXD0 pin before they are

    reconfigured. This would probably not be good if this pin is used for other purposes.