I recently acquired a Spark Fun RED-V Redboard with a FE310-G002 and have been working on some bare-metal programming with it. However, I have struggled to get the UARTs to work. I was poking through some of other peoples source code and noticed that the UART may need to be enabled through the GPIO. I see references to iof_en and iof_sel in the manual under section 17.8 HW I/O Functions (IOF). However, these registers are never mentioned anywhere else in the document. Their locations and contents are not defined in the manual as far as I can tell.
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You can find info in our bsp which is found at github.com/sifive/freedom-e-sdk. See for instance
https://github.com/sifive/freedom-e-sdk/blob/master/bsp/sifive-hifive1-revb/design.svd#L1012
There is also driver code in there which is presumably
https://github.com/sifive/freedom-metal/blob/bdeb3fd57b46bc30fceb1fb0ac3841ca19d23620/src/drivers/sifive_uart0.c
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