Serial Interface

The Serial Interface (or SI) is one of multiple I/O interfaces in the RCP, which is used to communicate with the PIF-NUS and in turn, Joybus devices.

Memory mapped registers are used to configure the Serial Interface and initiate DMA reads and writes. The base address for these registers is 0x0480 0000, also known as SI_BASE. However, because all memory accesses in the CPU are made using virtual addresses, the following addresses must be offset appropriately. For non-cached reads/writes, add 0xA000 0000 to the address. As an example, to directly write to the SI_DRAM_ADDR register, use address 0xA480 0000.

Registers

Table Notation:

R = Readable bit
W = Writable bit
U = Undefined/Unused bit
-n = Default value n at power on
[x:y] = Specifies bits x to y, inclusively

0x0480 0000 - SI_DRAM_ADDR

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SI_DRAM_ADDR 0x0480 0000
31:24	U-0	U-0	U-0	U-0	U-0	U-0	U-0	U-0
	—	—	—	—	—	—	—	—
23:16	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0
	DRAM_ADDR[23:16]
15:8	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0
	DRAM_ADDR[15:8]
7:0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0
	DRAM_ADDR[7:0]

bit 31-24	Undefined: Initialized to 0
bit 23-0	DRAM_ADDR[23:0]: RDRAM address used in SI DMAs

0x0480 0004 - SI_PIF_AD_RD64B

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SI_PIF_AD_RD64B 0x0480 0004

TODO

0x0480 0008 - SI_PIF_AD_WR4B

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SI_PIF_AD_WR4B 0x0480 0008

TODO

0x0480 0010 - SI_PIF_AD_WR64B

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SI_PIF_AD_WR64B 0x0480 0010

TODO

0x0480 0014 - SI_PIF_AD_RD4B

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SI_PIF_AD_RD4B 0x0480 0014
31:24	U-0	U-0	U-0	U-0	U-0	U-0	U-0	U-0
	—	—	—	—	—	—	—	—
23:16	U-0	U-0	U-0	U-0	U-0	U-0	U-0	U-0
	—	—	—	—	—	—	—	—
15:8	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0
	-	-	-	-	-	PIF_ADDR[10:8]
7:0	RW-0	RW-0	RW-0	RW-0	RW-0	RW-0	U-0	U-0
	PIF_ADDR[7:2]						0	0

bit 31-11	Undefined: Initialized to 0
bit 10-2	PIF_ADDR[10:2]: Address in PIF_ROM/PIF_RAM to read data from. Writing to this field should start the DMA transfer, but it actually doesn't work (nothing is transferred).

This register is broken and doesn't work. It was meant to execute a DMA transfer with a RD4B serial frame with PIF (to read 4 bytes from either PIF-ROM or PIF-RAM), but for some reason it does not work, and nothing is transferred. It is still documented here from completeness.

Notice that VR4300 can still do a 32-bit read from PIF-ROM/PIF-RAM using a direct memory read in the memory mapped space (physical address 0x1FC0 0000 - 0x1FC0 07FF and it will work correctly, though it will be a blocking read that will keep the VR4300 blocked as the serial transfer happens.

0x0480 0018 - SI_STATUS

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SI_STATUS 0x0480 0018
31:24	U-0	U-0	U-0	U-0	U-0	U-0	U-0	U-0
	—	—	—	—	—	—	—	—
23:16	U-0	U-0	U-0	U-0	U-0	U-0	U-0	U-0
	—	—	—	—	—	—	—	—
15:8	U-0	U-0	U-0	R-0	U-0	U-0	U-0	U-0
	—	—	—	INTERRUPT	DMA_STATE[3:0]
7:0	U-0	U-0	U-0	U-0	R-0	R-0	R-0	R-0
	PCH_STATE[3:0]				DMA_ERROR	READ_PENDING	IO_BUSY	DMA_BUSY

bit 31-13	Undefined: Initialized to 0
bit 12	INTERRUPT: Copy of SI interrupt flag from MIPS Interface, which is also seen in the RCP Interrupt Cause register.     Writing any value to SI_STATUS clears this bit in all three locations.     SI interrupts occur when either a DMA or IO write finishes.
bit 11-8	DMA_STATE[3:0]: Internal-only (likely not readable). Non-zero values indicate activity.
bit 7-4	PCH_STATE[3:0]: Internal-only (likely not readable). Non-zero values indicate activity.
bit 3	DMA_ERROR: Set when overlapping DMA requests occur. Can only be cleared with a power reset.
bit 2	READ_PENDING: Set when an IO read occurs, while an IO or DMA write is in progress.
bit 1	IO_BUSY: Set when a direct memory write to PIF_RAM is in progress.
bit 0	DMA_BUSY: Set when a read or write DMA, or an IO write, is in progress.