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docs:techref:hardware:port.gpio [2022/01/30 20:11] – multiplexig->multiplexing kenballusdocs:techref:hardware:port.gpio [2023/07/20 01:47] (current) – Add info box about gpiod-tools, gpio sysfs has been marked obsolete and won't work correctly djfe
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 Many times some GPIOs cannot be controlled by the kernel because they are multiplexed, and they are not acting as GPIOs. It might be possible to undo the multiplexing by writing some particular software programmable register settings, and then recover the GPIO functionality. But probably on most cases those GPIOs were multiplexed for some good reason, like using a [[docs:techref:hardware:soc|SoC]] pin  for a signal on an ethernet phy, SPI, PCI, UART or another purpose. Many times some GPIOs cannot be controlled by the kernel because they are multiplexed, and they are not acting as GPIOs. It might be possible to undo the multiplexing by writing some particular software programmable register settings, and then recover the GPIO functionality. But probably on most cases those GPIOs were multiplexed for some good reason, like using a [[docs:techref:hardware:soc|SoC]] pin  for a signal on an ethernet phy, SPI, PCI, UART or another purpose.
  
-  * Example 1: on the Broadcom BCM6328 SoC, GPIO pins 25, 26, 27 and 28 are used to indicate the LAN activity with hardware controlled LEDs. The memory register for setting this multiplexing is at 0x1000009C address, 64bits wide. Let's read it in OpenWrt\\ ''<color grey>root@OpenWrt:/# devmem 0x1000009C 64 +  * Example 1: on the Broadcom BCM6328 SoC, GPIO pins 25, 26, 27 and 28 are used to indicate the LAN activity with hardware controlled LEDs. The memory register for setting this multiplexing is at 0x1000009C address, 64bits wide. Let's read it in OpenWrt\\ <code>root@OpenWrt:/# devmem 0x1000009C 64 
-0x0154000000000000</color>''\\ {{:media:doc:hardware:bcm6328-lan-led_pinmux-bits.png?nolink|}}\\ These enabled bits are enabling every LAN LED to be controlled by hardware, therefore they cannot be controlled as regular GPIO's\\  Let's disable this multiplexing for all LEDs\\ ''<color grey>root@OpenWrt:/# devmem 0x1000009C 64 0x0</color>''\\ Now we can export these GPIOs and control them via software.+0x0154000000000000</code>\\ {{:media:doc:hardware:bcm6328-lan-led_pinmux-bits.png?nolink|}}\\ These enabled bits are enabling every LAN LED to be controlled by hardware, therefore they cannot be controlled as regular GPIO's\\  Let's disable this multiplexing for all LEDs\\ <code>root@OpenWrt:/# devmem 0x1000009C 64 0x0</code>\\ Now we can export these GPIOs and control them via software.
  
-  * Example 2: on the Atheros AR7240 SoC, GPIO pins 6, 7 and 8 are used by the JTAG interface. The memory register for AR7240 GPIO pinmux is at 0x18040028 address, 32 bits wide. Let's read it:\\ ''<color grey>root@OpenWrt:/# devmem 0x18040028 32 +  * Example 2: on the Atheros AR7240 SoC, GPIO pins 6, 7 and 8 are used by the JTAG interface. The memory register for AR7240 GPIO pinmux is at 0x18040028 address, 32 bits wide. Let's read it:\\ <code>root@OpenWrt:/# devmem 0x18040028 32 
-0x48002</color>''\\ The bit0 disables JTAG multiplexing, then we must set this bit to 1, in other words, write the value 0x48003\\ ''<color grey>root@OpenWrt:/# devmem 0x18040028 32 0x48003</color>''\\ Now we can control these GPIO pins (GPIO6, GPIO7 and GPIO8)+0x48002</code>\\ The bit0 disables JTAG multiplexing, then we must set this bit to 1, in other words, write the value 0x48003\\ <code>root@OpenWrt:/# devmem 0x18040028 32 0x48003</code>\\ Now we can control these GPIO pins (GPIO6, GPIO7 and GPIO8)
 ==== GPIO Interrupts ==== ==== GPIO Interrupts ====
 GPIO interrupts are useful when a GPIO is used as input and you need to manage high signal frequencies. Without interrupts, GPIO inputs must be managed using the **polling** method. With polling you cannot manage signal inputs with high frequencies, but still can be used for simple tasks like reading the input on the device buttons.  GPIO interrupts are useful when a GPIO is used as input and you need to manage high signal frequencies. Without interrupts, GPIO inputs must be managed using the **polling** method. With polling you cannot manage signal inputs with high frequencies, but still can be used for simple tasks like reading the input on the device buttons. 
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 ===== Software ===== ===== Software =====
 +<WRAP round info 100%>
 +**The GPIO SYSFS interface has been marked as obsolete in 2015. I couldn't get the following scripts to work. This page needs an overhaul.**
 +
 +The new interface is the linux GPIO character device. One way to access it is [[https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/tree/README|libgpiod]].
 +
 +You can install it with the [[:packages:pkgdata:gpiod-tools]] package like this:
 +
 +<code bash>opkg update
 +opkg install gpiod-tools</code>
 +
 +A good article on how to use the new CLI and why we should move on: [[https://www.thegoodpenguin.co.uk/blog/stop-using-sys-class-gpio-its-deprecated/|Stop using /sys/class/gpio – it’s deprecated]]
 +
 +The biggest differences are:
 +  * The new interface isn't stateless like the old one. Processes are able to take exclusive control over a gpio line now
 +  * There is a character device /dev/gpiochipX for each gpio chip found in the system
 +  * TODO
 +As I said: This page needs an overhaul. This info box is just to prevent people from wasting their time on a deprecated ABI.
 +</WRAP>
 +
 In linux GPIOs can be accessed through GPIO SYSFS interface: **/sys/class/gpio/** In linux GPIOs can be accessed through GPIO SYSFS interface: **/sys/class/gpio/**
  
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   - Compare the changed GPIO value which corresponding to the button held   - Compare the changed GPIO value which corresponding to the button held
 ===== Links ===== ===== Links =====
-  * [[http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/gpio|Kernel gpio documentation]]+  * [[https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/driver-api/gpio|Kernel gpio documentation]]
   * [[http://www.mjmwired.net/kernel/Documentation/gpio.txt|other kernel gpio doc]]   * [[http://www.mjmwired.net/kernel/Documentation/gpio.txt|other kernel gpio doc]]
  
  • Last modified: 2022/01/30 20:11
  • by kenballus