ST7285C datasheet(61/117 Pages) STMICROELECTRONICS

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ST7285C

2C BUS INTERFACE (Cont’d)

STATUS REGISTER 2 (SR2)

Address: 002Ah

—

Read Only

Reset Value: 00h

b7-5 = reserved.

b4: AF

Acknowledge Failure

The Acknowledge Failure bit is set when no ac-

knowledge is returned. If this bit is set, then an in-

terrupt is sent to the microcontroller if ITE is set.

During this interrupt, the SCL line is not hold low.

This bit is cleared by a read of the Status Register.

It is also cleared when the peripheral is disabled

(PE=0) or by reset.

b3: STOPF

Stop Detection Flag (in Slave mode)

StopF bit is set when a Stop condition is detected

on the SCL line after an acknowledge of byte.

When this bit is set, an interrupt is sent to the mi-

crocontroller if ITE is set. During this interrupt, the

SCL line is not hold low.

This bit is cleared by a read of the status register

(when StopF is set). It is also cleared when the pe-

ripheral is disabled (PE=0) or by reset.

b2: ARLO

Arbitration Lost

ARLO is set when the I

2C interface loses the arbi-

tration of the bus to another Master. After ARLO is

set, the interface operates in Slave mode (M/SL at

a logic low) and an interrupt is generated if ITE is

set. During this interrupt, the SCL line is not hold

low.

This bit is cleared by a read of the Status register.

It is also cleared when the peripheral is disabled

(PE=0) or by reset.

b1: BERR

Bus Error

BERR bit is set when a misplaced start or stop

condition is detected. If this bit is set, then an inter-

rupt is sent to the microcontroller if ITE is set. Dur-

ing this interrupt, the SCL line is not hold low.

The Bus Error flag bit is cleared by a read of the

status register (when BERR is set). It is also

cleared when the peripheral is disabled (PE=0) or

by reset.

b0: GCAL

General Call (Slave mode)

If ENGC is set, GCAL is set following detection of

a general call address.

It is cleared by the detection of a stop condition, by

reset or when the peripheral is disabled (PE=0).

4.5.6 I

2C State Machine:

In I

2C mode, the I2C interface always operates in

Slave mode (M/SL at logic low level) except when

it initiates a transmission or a receive sequence.

It enables the multimaster function with an auto-

matic switch from Master mode to Slave mode

when the interface loses the arbitration of the I

bus. So, the Slave process is active both in Slave

mode and in Master mode.

4.5.6.1 Slave mode

As soon as a start condition is detected, the ad-

dress word is received from the SDA line and it is

sent to the shift register; then it is compared with

the interface address.

– Address no matched: the state machine is reset

and it waits for another Start bit.

– Address matched: the Addressed As Slave bit

(ADSL) is set and an acknowledge bit is sent to

the Master if ACK is set. So an interrupt is sent to

the microcontroller if ITE is set; it then waits for

the microcontroller to read Status Register 1 by

holding the SCL line low.

Then, depending on the Data Direction bit (least

significant bit), and after generating an acknowl-

edge, the Slave must enter Send or Receive

mode.

4.5.6.2 Slave Receiving

The Slave receives words from the SDA line into

the shift register and it sends them to the data reg-

ister. After each word it generates an acknowledge

bit if the Enable Acknowledge flag is set. When the

acknowledge bit is sent, the BTF flag is set and an

interrupt is generated if ITE is set.Then it waits for

the microcontroller to read the Data Register by

holding the SCL line low.

- Detection of a Stop or a Start condition during a

byte reception: the BERR flag is set and an inter-

rupt is generated.

- Detection of a Start condition after an acknowl-

edge time-slot: the state machine is reset and

starts a new process.

- Detection of a Stop condition after an acknowl-

edge time-slot: the Slave state machine is reset.

Then the SSTOP flag is set and an interrupt is

generated if ITE is set.

- The Stop bit is set in the control register: the state

machine is reset after transfer of the current byte.

STOP F

ARLO

BERR

GCAL

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