UPSD3422 datasheet(124/293 Pages) STMICROELECTRONICS

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I2C interface

uPSD34xx

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23.10

I2C data shift register (S1DAT)

The S1ADR register (Table 57) holds a byte of serial data to be transmitted or it holds a

serial byte that has just been received. The MCU may access S1DAT while the SIOE is not

in the process of shifting a byte (the INTR flag indicates shifting is complete).

While transmitting, bytes are shifted out MSB first, and when receiving, bytes are shifted in

MSB first, through the Acknowledge Bit register as shown in Figure 43 on page 120.

23.10.1

Bus wait condition

After the SIOE finishes receiving a byte in Receive mode, or transmitting a byte in Transmit

mode, the INTR flag (in S1STA) is set and automatically a wait condition is imposed on the

I2C bus (SCL held low by SIOE). In Transmit mode, this wait condition is released as soon

as the MCU writes any byte to S1DAT. In Receive mode, the wait condition is released as

soon as the MCU reads the S1DAT register.

This method allows the user to handle transmit and receive operations within an interrupt

service routine. The SIOE will automatically stall the I2C bus at the appropriate time, giving

the MCU time to get the next byte ready to transmit or time to read the byte that was just

received.

Table 57.

S1DAT: I2C data shift register (SFR DEh, reset value 00h)

23.11

I2C address register (S1ADR)

The S1ADR register (Table 58) holds the 7-bit device address used when the SIOE is

operating as a Slave. When the SIOE receives an address from a Master, it will compare

this address to the contents of S1ADR, as shown in Figure 43 on page 120.

If the 7 bits match, the INTR Interrupt flag (in S1STA) is set, and the ADDR Bit (in S1CON) is

set. The SIOE cannot modify the contents S1ADR, and S1ADR is not used during Master

mode.

Table 58.

S1ADR: I2C address register (SFR DFh, reset value 00h)

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

S1DAT[7:0]

Bit

Symbol

R/W

Function

7:0

S1DAT[7:0]

R/W

Holds the data byte to be transmitted in Transmit mode, or it

holds the data byte received in Receiver mode.

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

SLA6

SLA5

SLA4

SLA3

SLA2

SLA1

SLA0

–

Bit

Symbol

R/W

Function

7:1

SLA[6:0]

R/W

Stores desired 7-bit device address, used when SIOE is in

Slave mode.

–

Not used

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