How to Choose a GPIO to Use

You should be able to use an GPIO that is not already in use. Conflicting PinMuxing will results in errors. Refer to this FAQ for more information: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1494485/faq-processor-sdk-getting-started-with-gpios-in-linux-using-the-am62-and-am64-family-processors

Adding the Device and Clocks to R5 SPL so the GPIO can be used

In the Linux SDK, navigate to <SDK Install Path>/board-support/ti-u-boot-<version>/arch/arm/mach-k3/r5/<device>/.

The two files will be "dev-data.c" and "clk-data.c".

The file 'dev-data.c' will contain information about the devices that will be booted. The file 'clk-data.c' will contain information about the clocks that will be utilized and muxed.

Make sure that the device and clocks that need to be used are in the structs in each of these files.

For more information about defining the devices and clocks, refer to the TISCI API section for your device: https://downloads.ti.com/tisci/esd/latest/5_soc_doc/index.html

Configure the R5 SPL init to set the GPIO

Navigate to <SDK Install Path>/board-support/ti-u-boot-<version>/arch/arm/mach-k3/<device>_init.c

At the bottom of the initialization code sequence (after all the PLLs and Memory mapped registers are set), we can directly write to the registers.

For the quickest way to set the GPIO, do the following:

• Set the PADCONFIG register to enable the TX/RX bits for the pad.
  • Refer to the device's datasheet to determine the PADCONFIG register address.
  • Refer to the TRM's PADCONFIG Register section for more details about the register itself
• Set the GPIO state (high or low) using the memory mapped register
  • Refer to GPIO Registers in the device's TRM for more information about all the registers
• Set the GPIO direction (input or output) using the memory mapped register.

We can execute this sequence on AM62x using GPIO0_38.

From the datasheet, we can see GPIO0_38 uses PADCONFIG39 (address 0x000F409C).

We can write to this address to set the Mux Mode to 7, TX enabled and RX enabled (0x00050007). Refer to this FAQ for more information about PADCONFIGS: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1494485/faq-processor-sdk-getting-started-with-gpios-in-linux-using-the-am62-and-am64-family-processors

Then we can write to the GPIO MMR registers.

GPIO0_38 falls under Bank 2, Pin 6 (38/16 = 2 remainder 6)

This means the GPIO MMR registers will be for Bank 2/3 and pin 6.

To set the GPIO status high, set the status register bit 6 to high. To set the direction (1 is input, 0 is output), set the direction register bit 6 to low.

This example is for AM62x where the sequence is executed.

diff --git a/arch/arm/mach-k3/am625_init.c b/arch/arm/mach-k3/am625_init.c
index 8238ca3a..9c71e55b 100644
--- a/arch/arm/mach-k3/am625_init.c
+++ b/arch/arm/mach-k3/am625_init.c
@@ -222,6 +222,12 @@ void board_init_f(ulong dummy)
        }
 
        spl_enable_cache();
+
+       printf("lets try to write a GPIO\n");
+       writel(0x00050007, 0x000F409C); //write the padconfig
+       writel(0x40,0x600040); //set GPIO status to high
+       writel(0xFFFFFFBF,0x600038); //set GPIO direction or just write 0 to all
+       printf("end of GPIO test\n");
 }

Compiling U-Boot

After these changes are made, u-boot needs to be recompiled. Refer to the Linux SDK documentation to re-compile U-Boot

AM62A: https://software-dl.ti.com/processor-sdk-linux/esd/AM62AX/latest/exports/docs/linux/Foundational_Components/U-Boot/UG-General-Info.html

AM62P: https://software-dl.ti.com/processor-sdk-linux/esd/AM62PX/latest/exports/docs/linux/Foundational_Components/U-Boot/UG-General-Info.html

AM62x: https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/latest/exports/docs/linux/Foundational_Components/U-Boot/UG-General-Info.html

Load the new boot images for the boot mode used.

Signal Trace

The signal remains high after U-Boot has completed and Linux has successfully booted.