How does gpiod number the GPIO pins?

Question

With gpiod installed on a Raspberry Pi 5, you can use gpioinfo to list all the GPIO lines:

$ sudo gpioinfo gpiochip4
gpiochip4 - 54 lines:
    line   0:      "ID_SD"       unused   input  active-high 
    line   1:      "ID_SC"       unused   input  active-high 
    line   2:       "PIN3"       unused   input  active-high 
    line   3:       "PIN5"       unused   input  active-high 
    line   4:       "PIN7"       unused   input  active-high 
    line   5:      "PIN29"       unused   input  active-high 
    line   6:      "PIN31"       unused   input  active-high 
    line   7:      "PIN26"   "spi0 CS1"  output   active-low [used]
    line   8:      "PIN24"   "spi0 CS0"  output   active-low [used]
    line   9:      "PIN21"       unused   input  active-high 
    line  10:      "PIN19"       unused   input  active-high 
    line  11:      "PIN23"       unused   input  active-high 
    line  12:      "PIN32"       unused   input  active-high 
    line  13:      "PIN33"       unused   input  active-high 
    line  14:       "PIN8"       unused   input  active-high 
    line  15:      "PIN10"       unused   input  active-high 
    line  16:      "PIN36"       unused   input  active-high 
    line  17:      "PIN11"       unused   input  active-high 
    line  18:      "PIN12"       unused   input  active-high 
    line  19:      "PIN35"       unused   input  active-high 
    line  20:      "PIN38"       unused   input  active-high 
    line  21:      "PIN40"       unused   input  active-high 
    line  22:      "PIN15"       unused   input  active-high 
    line  23:      "PIN16"       unused   input  active-high 
    line  24:      "PIN18"       unused   input  active-high 
    line  25:      "PIN22"       unused   input  active-high 
    line  26:      "PIN37"       unused   input  active-high 
    line  27:      "PIN13"       unused   input  active-high 
    line  28: "PCIE_RP1_WAKE" unused input active-high 
    line  29:   "FAN_TACH"       unused   input  active-high 
    line  30:   "HOST_SDA"       unused   input  active-high 
    line  31:   "HOST_SCL"       unused   input  active-high 
    line  32:  "ETH_RST_N"  "phy-reset"  output   active-low [used]
    line  33:          "-"       unused   input  active-high 
    line  34: "CD0_IO0_MICCLK" "cam0_reg" output active-high [used]
    line  35: "CD0_IO0_MICDAT0" unused input active-high 
    line  36: "RP1_PCIE_CLKREQ_N" unused input active-high 
    line  37:          "-"       unused   input  active-high 
    line  38:    "CD0_SDA"       unused   input  active-high 
    line  39:    "CD0_SCL"       unused   input  active-high 
    line  40:    "CD1_SDA"       unused   input  active-high 
    line  41:    "CD1_SCL"       unused   input  active-high 
    line  42: "USB_VBUS_EN" unused output active-high 
    line  43:   "USB_OC_N"       unused   input  active-high 
    line  44: "RP1_STAT_LED" "PWR" output active-low [used]
    line  45:    "FAN_PWM"       unused  output  active-high 
    line  46: "CD1_IO0_MICCLK" "cam1_reg" output active-high [used]
    line  47:  "2712_WAKE"       unused   input  active-high 
    line  48: "CD1_IO1_MICDAT1" unused input active-high 
    line  49: "EN_MAX_USB_CUR" unused output active-high 
    line  50:          "-"       unused   input  active-high 
    line  51:          "-"       unused   input  active-high 
    line  52:          "-"       unused   input  active-high 
    line  53:          "-"       unused   input  active-high

The mapping of these lines to the BCM pin numbers is non-obvious. Is PIN5 referring to BCM pin 5 (physical pin 29)? Or is PIN29 referring to physical pin 29 (BCM pin 5)?

rgov · Accepted Answer · 2024-03-18T04:52:27.073

The line numbers 0-27 correspond with the BCM numbers of the GPIOs. This numbering scheme is what you will virtually always use in software.

(Note: The gpioget tool uses another term, "offset", which means the same as line number, and in the context of RPi, BCM number.)

As @Milliways points out, this numbering is intrinsic to the Broadcom SoC design, where our software is running, whereas the way these are wired the 40-pin header is arbitrarily decided by the Raspberry Pi designers.

The Raspberry Pi device tree helpfully defines gpio-line-names which give textual labels to the GPIO lines. In these labels, PIN# are referring to the physical pin numbers. For the Raspberry Pi 5, these are:

3, 5, 7, 8, 10, 11, 12, 13, 15, 16,
18, 19, 21, 22, 23, 24, 26, 29, 31,
32, 33, 35, 36, 37, 38, 40

For example, GPIO 25 appears as line 25: "PIN22"; the label lets us know it is physical pin 22.

Note that GPIO 0 (physical pin 27) is labeled ID_SD, and GPIO 1 (physical pin 28) is labeled ID_SC. These are normally reserved for querying the EEPROM of an attached HAT.

To conclude, to use gpioget you must use the BCM aka line number aka "offset".

How does gpiod number the GPIO pins?

1 Answers1