Reduced Serial Bus

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The RSBTM is a push-pull two wire bus developed by Allwinner Technology. Its primary (only?) use is to connect an (Allwinner) SoC to an AXP-series power management controller (PMIC) from X-Powers.


  • Related to the SMBus, which in turn is derived from I²C.
  • Connects to the SoC-internal AMBA Peripheral Bus (APB).
  • Supports speeds up to 20 MHz (in contrast to the 400 KHz of I²C).
  • Supports multiple devices, although most (all?) boards connect the pins to the only on-board PMIC.
  • Multiplexed with some I²C pins (allows driving slaves with I²C for compatibility).
  • Supports programmable output delay of CD signal.
  • Supports parity check for address and data transmission.
  • Included in A23, A33, A80, A83T, H3(?) and A64 SoCs.


The RSB is briefly mentioned (along with MMIO address and IRQ number) in the A23, A33 and A64 manuals, but explained in more detail (including a register description) in the A80 and A83 manuals.


This list was originally gained from Allwinner code, but was later extended with the help of the manuals.

Register list

Offset Name Description
0x00 CTRL Control
0x04 CCR Clock control
0x08 INTE Interrupt enable
0x0C STAT Interrupt status (write 1 to clear)
0x10 DADDR0 Register address within the slave
0x14 DADDR1
0x18 DLEN (Not sure if this exists)
0x1C DATA0 Up to four data bytes
0x20 DATA1
0x24 LCR Line control register
0x28 PMCR PMIC init register
0x2C CMD Command for next transaction
0x30 SADDR Slave address

The DLEN register seems to be hardwired to 0 in most recent implementations and is redundant with the CMD register. According to documentation it holds the transfer length (1, 2 or 4 Bytes) in the lower two bits and the read(=1)/write(=0) indicator in bit 4.


Bit Name Description Value R/W Default
31-9 Reserved
8 USE_RSB Use RSB interface RW 0
7 START_TRANS Start transfer write 1 to start current transfer, resets to 0 when done RW 0
6 ABT_TRANS Abort transfer write 1 to abort current transfer, resets to 0 when done RW 0
5-2 Reserverd
1 GLB_INTEN Enable interrupts 1: enable RW 0
0 SOFT_RESET Reset RSB controller, resets to 0 when done write 1 to reset RW 0


This register controls the RSB bus clock (SCK) speed.

Bit Name Description Value R/W Default
31-9 Reserved
8 CD_ODLY ?? cd_odly = !(div >> 1) RW
7-0 DIV Clock divider SCK = SRC / 2 / (DIV + 1) RW

The reference clock is the 24 MHz oscillator, a common speed on most boards seems to be 3 MHz, so 0x103 is the value you most probably want to write into this register.


This register masks/unmasks interrupt events.

Bit Name Description R/W Default
31-3 Reserved
2 LBSY Load busy (transfer in progress) RW
1 TERR Transfer error encountered RW
0 TOVER Transfer over (completed) RW


This register shows interrupt and transfer status. Write 1 to the corresponding bit to clear each interrupt.

Bit Name Description R/W Default
31-24 Reserved
23-16 TRANS_ERR_ID Transfer error ID for transfer 0 RW
15-8 TRANS_ERR_ID Transfer error ID for transfer 0 RW
2 LBSY Load busy (transfer in progress) RW
1 TERR Transfer error encountered RW
0 TOVER Transfer over (completed) RW

Transfer error codes

Bit Description
15-9 Reserved
8 No ACK when setting run-time slave address
7-4 Reserved
3 Error happened with the transmission of the 4th byte of data
2 Error happened with the transmission of the 3th byte of data
1 Error happened with the transmission of the 2th byte of data
0 Error happened with the transmission of the 1th byte of data


This register can seemingly be used to bit bang the bus.


Educated guess: This register can be used to switch a PMIC from its reset-state configured I2C interface to RSB mode. This avoids configuring the pins to I2C and sending the switch sequence via standard I2C.


List of command codes

Name Value Description
WR8 0x4E Write byte
WR16 0x59 Write half word
WR32 0x63 Write word
RD8 0x8B Read byte
RD16 0x9C Read half word
RD32 0xA6 Read word
SRTA 0xE8 Set run-time address


This register is used with the SRTA command to connect a hardware address to a runtime address.

Bit Description
23-16 Run-Time Slave Address
15-0 Hardware Slave Address

Hardware Addresses

RSB has hardware addresses and runtime addresses. Runtime addresses are configured at initialization time, and are used to talk to the slave devices. Hardware addresses are used to identify and configure runtime addresses. The configured runtime addresses are not queryable.

Allwinner sources use a static mapping of hardware and runtime addresses.

Hardware Address (Static) Runtime Address Known Devices
0x3a3 0x2d AXP223, AXP809, AXP81X
0x745 0x3a AXP806
0x3a3 0x2d AXP803
0xe89 0x4e AC100

In contrast to the AXP 803 manual, the hardware address for it seems to be 0x3a3 instead of the documented 0x1d1.

Example transactions

For setting up the RSB controller in the first place, you have to:

  • Configure the associated pins to connect to the RSB controller (Port Controller CPUs)
  • Configure the pins to be Pull-up level 2 drive strength (same Port Controller CPUs)
  • Un-gate the RSB clock (bit 3 in the APB0_CLK_GATING_REG (offset 0x28) in the R_PRCM block)
  • De-assert the RSB reset line (bit 3 in the APB0_SOFT_RST_REG (offset 0xB0) in the R_PRCM block)
  • Program the clock (CCR register) in the RSB block (1 cycle delay, 3 MHz)
  • Soft reset the RSB block (bit 0 in CTRL register)

After having setup the RSB block, a typical read transaction may look like:

writel(rt_addr << 16, RSB_BASE + 0x30); /* set run-time address of slave */
writel(0x8b, RSB_BASE + 0x2c);          /* "Read-one-byte" command into CMD register */
writel(regnr, RSB_BASE + 0x10);         /* set register to be read */
writel(0x80, RSB_BASE + 0x00);          /* start transaction */
while (readl(RSB_BASE + 0x00) & 0x80)   /* poll for completion */
ret = readl(RSB_BASE + 0x0c);           /* read status register */
if (ret == 0x01)                        /* transaction completed without errors */
        return 0;
return ret;