SPI

SPI and SPI-flash access are implemented in the Flash Interface Unit (FIU).

Although NPCM750's SPI controller is also called FIU, it uses an incompatible register layout.

physical address	description
0x40000000	SPI flash access window
0xc0000000	SPI flash access window (alias)
0xc8000000	SPI controller registers

An unrelated secondary SPI controller also exists, but isn't described here.

Registers

FIU has the following registers at 0xc8000000:

offset	type	name	description
0x00	u8	FIU_CFG	FIU configuration
0x01	u8	BURST_CFG	Burst configuration
0x02	u8	RESP_CFG	FIU response configuration
0x03	u8	CFBB_PROT	Core firmware boot block protection
0x04	u16	FWIN1_LOW	Flash Access Window 1, low limit
0x06	u16	FWIN1_HIGH	Flash Access Window 1, high limit
0x08	u16	FWIN2_LOW	Flash Access Window 2, low limit
0x0a	u16	FWIN2_HIGH	Flash Access Window 2, high limit
0x0c	u16	FWIN3_LOW	Flash Access Window 3, low limit
0x0e	u16	FWIN3_HIGH	Flash Access Window 3, high limit
0x10	u8	PROT_LOCK	Protection lock
0x11	u8	PROT_CLEAR	Protection and lock clear
0x14	u8	SPI_FL_CFG	SPI flash configuration
0x16	u8	UMA_CODE	UMA code byte
0x17	u8	UMA_AB0	UMA Address Byte 0, LSB, transmitted last
0x18	u8	UMA_AB1	UMA Address Byte 1, transmitted second
0x19	u8	UMA_AB2	UMA Address Byte 2, MSB, transmitted first
0x1a	u8	UMA_DB0	UMA Data Byte 0
0x1b	u8	UMA_DB1	UMA Data Byte 1
0x1c	u8	UMA_DB2	UMA Data Byte 2
0x1d	u8	UMA_DB3	UMA Data Byte 3
0x1e	u8	UMA_CTS	UMA control and status
0x1f	u8	UMA_ECTS	UMA extended control and status

FIU_CFG - FIU configuration

This 8-bit field contains the size of the flash (in units of 512KiB).

Beyond the configured size, reads to the MMIO windows at 0x40000000 or 0xc0000000 will return 0xff.

BURST_CFG - Burst configuration

bits	description	values
5:4	write burst
3:2	read burst	0 = 1 byte, 3 = 16 bytes

Read burst

When the read burst size is set to 1, SPI reads read only one byte at a time. When it is set to 16, then longer transactions are possible, but 1/2/4 byte fetches will still only read that many bytes from the SPI flash.

RESP_CFG - FIU response configuration

bits	description
1	INT_EN
0	IAD_EN

FWIN* - Flash Access Window configuration

The flash access windows are configured through FWINx_LOW and FWINx_HIGH, in units of 4096 bytes, but their purpose is unknown: Read accesss through 0x40000000 is still available after setting all windows to 0:0.

NOTE: Writes to the window registers must be 16-bit or 32-bit writes. 8-bit writes are apparently discarded.

UMA_CTS - UMA control and status

bits	mask	description
7	0x80	write 1: trigger command execution; read 1: busy
6:5	0x60	chip select number (0-3)
4	0x10	SPI direction: read (0) or write (1); exact function to be confirmed.
3	0x08	use address field
2:0	0x07	number of data bytes (0-4)

UMA_ECTS - UMA extended control and status

bits	description
3	chip select 3 (0=assert, 1=deassert)
2	chip select 2 (0=assert, 1=deassert)
1	chip select 1 (0=assert, 1=deassert)
0	chip select 0 (0=assert, 1=deassert)

This register is used to assert the chip select lines manually.

Automatic dummy byte insertion (for fast reads)

In UMA transfers, the controller automatically inserts a dummy byte into the SPI transfer, if all of the following conditions are met:

• UMA_CODE is set to 0x0b (fast read)
• UMA_CTS.RD_WR is set to 0 (read)
• UMA_CTS.A_SIZE is set to 1 (use 3 address bytes)
• UMA_CTS.D_SIZE is set to 1-4 (use 1-4 data bytes)

code	R/W	A	D	dummy byte inserted?
0x0b	R	3	1-4	yes
0x0b	W	*	*	no
0x0b	*	0	*	no
0x0b	*	*	0	no
other	*	*	*	no

Limitations

Four byte addressing

The FIU was not designed with 4-byte addressing in mind, but we can work around this fact, by chaining multiple transfers together (see UMA_ECTS).

READ 4B:

Flash view:  [ C  A  A  A   A     D  D  D  D]
bytes:        13 aa bb cc  dd -> 5a a5 f0 0f
FIU's view:  [ C  A  A  A][ C     D  D  D  D]
FIU mode:    [ read/write][      read       ]

Even if the last address byte is 0x0b, a dummy byte will not be inserted because the second transfer (from FIU's point of view) doesn't have address bytes.

FAST READ 4B:

Flash view:  [ C  A  A  A  A  Du     D  D  D  D]
bytes:        0c aa bb cc dd  00 -> 5a a5 f0 0f
FIU's view:  [ C  A  A  A  D][ C     D  D  D  D]
FIU mode:    [    write     ][      read       ]

Long reads

Due to the structure of a command as represented in FIU, only up to four bytes can be read at most. This presents a challenge in a few cases.

RDID (read identification)

Although most flash memories can be identified by the first 3 bytes returned by the RDID command (0x9f), Linux reads 6 bytes.

FIU does not allow gapless reading of more than four bytes at once. However, because the device ID is constant, we can read it twice and stitch the bytes together.

Flash view:  [ C     I  I  I ]
bytes:        9f -> aa bb cc ]
FIU's view:  [ C     D  D  D ]
FIU mode:    [       read    ]

Flash view:  [ C     I  I  I  I  I  I ]
bytes:        9f -> xx xx xx dd ee ff ]
FIU's view:  [ C     A  A  A  D  D  D ]
FIU mode:    [          read          ]

References

• Drivers: ATEN, AMI, Dell (header)