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Understanding the NXP i.MX6UL Pin Mux (Part 2)

In the previous post, it was noted that bit 30 needs to be set in the i.MX6UL pad config if you want to read the state of a GPIO output. Digging into this a bit more, we find the following text in the Documentation/devicetree/bindings/pinctrl/fsl,imx-pinctrl.txt file:

SION(1 << 30): Software Input On Field.
Force the selected mux mode input path no matter of MUX_MODE functionality. By default the input path is determined by functionality of the selected mux mode (regular).

In the i.MX6UL developers manual, we also find the following:

When we look at the SW_MUX_CTL register, we notice the SION bit is located at the bit 4 position.

So the question is: How does bit 30 in the device tree file get mapped to bit 4 in the SW_MUX_CTL register? In the Linux kernel source we find the following:


/* The bits in CONFIG cell defined in binding doc*/
#define IMX_NO_PAD_CTL 0x80000000 /* no pin config need */
#define IMX_PAD_SION 0x40000000 /* set SION */


 /* SION bit is in mux register */
 config = be32_to_cpu(*list++);
 if (config & IMX_PAD_SION)
 pin->mux_mode |= IOMUXC_CONFIG_SION;
 pin->config = config & ~IMX_PAD_SION;


#define IOMUXC_CONFIG_SION (0x1 << 4)

From this code, we can see that bit 30 is a virtual bit that gets translated into bit 4 in the mux mode register.

Hope this helps in understanding how to configure the i.MX6UL gpio. Once you understand the details; the process is very simple.