ST72F521 datasheet(118/215 Pages) STMICROELECTRONICS

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ST72F521, ST72521B

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I2C BUS INTERFACE (Cont’d)

10.7.4 Functional Description

Refer to the CR, SR1 and SR2 registers in Section

10.7.7. for the bit definitions.

By default the I2C interface operates in Slave

mode (M/SL bit is cleared) except when it initiates

a transmit or receive sequence.

First the interface frequency must be configured

using the FRi bits in the OAR2 register.

10.7.4.1 Slave Mode

As soon as a start condition is detected, the

address is received from the SDA line and sent to

the shift register; then it is compared with the

address of the interface or the General Call

address (if selected by software).

Note: In 10-bit addressing mode, the comparison

includes the header sequence (11110xx0) and the

two most significant bits of the address.

Header matched (10-bit mode only): the interface

generates an acknowledge pulse if the ACK bit is

set.

Address not matched: the interface ignores it

and waits for another Start condition.

Address matched: the interface generates in se-

quence:

– Acknowledge pulse if the ACK bit is set.

– EVF and ADSL bits are set with an interrupt if the

ITE bit is set.

Then the interface waits for a read of the SR1 reg-

ister, holding the SCL line low (see Figure 66

Transfer sequencing EV1).

Next, in 7-bit mode read the DR register to deter-

mine from the least significant bit (Data Direction

Bit) if the slave must enter Receiver or Transmitter

mode.

In 10-bit mode, after receiving the address se-

quence the slave is always in receive mode. It will

enter transmit mode on receiving a repeated Start

condition followed by the header sequence with

matching address bits and the least significant bit

set (11110xx1).

Slave Receiver

Following the address reception and after SR1

from the SDA line into the DR register via the inter-

nal shift register. After each byte the interface gen-

erates in sequence:

– Acknowledge pulse if the ACK bit is set

– EVF and BTF bits are set with an interrupt if the

ITE bit is set.

Then the interface waits for a read of the SR1 reg-

ister followed by a read of the DR register, holding

the SCL line low (see Figure 66 Transfer se-

quencing EV2).

Slave Transmitter

Following the address reception and after SR1

the DR register to the SDA line via the internal shift

The slave waits for a read of the SR1 register fol-

lowed by a write in the DR register, holding the

SCL line low (see Figure 66 Transfer sequencing

EV3).

When the acknowledge pulse is received:

– The EVF and BTF bits are set by hardware with

an interrupt if the ITE bit is set.

Closing slave communication

After the last data byte is transferred a Stop Con-

dition is generated by the master. The interface

detects this condition and sets:

– EVF and STOPF bits with an interrupt if the ITE

bit is set.

Then the interface waits for a read of the SR2 reg-

ister (see Figure 66 Transfer sequencing EV4).

Error Cases

– BERR: Detection of a Stop or a Start condition

during a byte transfer. In this case, the EVF and

the BERR bits are set with an interrupt if the ITE

bit is set.

If it is a Stop then the interface discards the data,

released the lines and waits for another Start

condition.

If it is a Start then the interface discards the data

and waits for the next slave address on the bus.

– AF: Detection of a non-acknowledge bit. In this

case, the EVF and AF bits are set with an inter-

rupt if the ITE bit is set.

The AF bit is cleared by reading the I2CSR2 reg-

ister. However, if read before the completion of

the transmission, the AF flag will be set again,

thus possibly generating a new interrupt. Soft-

ware must ensure either that the SCL line is back

at 0 before reading the SR2 register, or be able

to correctly handle a second interrupt during the

9th pulse of a transmitted byte.

Note: In case of errors, SCL line is not held low;

however, the SDA line can remain low if the last

bits transmitted are all 0. While AF=1, the SCL line

may be held low due to SB or BTF flags that are

set at the same time. It is then necessary to re-

lease both lines by software.

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