Saved FAQ from Freescale.com for information only:
Question. Please clarify how to use the SION bit ?
Answer. The SION, Software Input ON, bit field on many I/O pads is shortly described in section A.3.2 on page A-6 of the iMX51 reference manual.
Typically, an I/O pad is configured as either an Input or an Output pin but some pads are dynamically switched the functions in time such as the SD_CMD pad. This pad is used to send SD commands to the card and then immediately used to read the card's response back.
In this situation, it is better to have the IOMUX connects the pad to both command and response of the SD/MMC block then the software does not have to switch the MUX_MODE of that pad. In this case the SION bit field is set so the SD Commands can go out on the pad and the Responses from the card can be simultaneously read back.
Another use for this SION field is to set this bit to have a feedback on a GPIO pad. The software can read the state of the output pad to see whether there is an abnormal behavior on the pad. You can set this SION bit on any pad that has it to have a feedback of the pad independent of the MUX_MODE bits. The purpose of this bit is to force connection of the pad to the read the state of the pad.
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