ATmega32A datasheet(199/352 Pages) ATMEL

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8155B–AVR–07/09

ATmega32A

Figure 20-18. Formats and States in the Slave Transmitter Mode

20.7.5

Miscellaneous States

There are two status codes that do not correspond to a defined TWI state, see Table 20-6.

Status $F8 indicates that no relevant information is available because the TWINT Flag is not set.

This occurs between other states, and when the TWI is not involved in a serial transfer.

Status $00 indicates that a bus error has occurred during a Two-wire Serial Bus transfer. A bus

error occurs when a START or STOP condition occurs at an illegal position in the format frame.

Examples of such illegal positions are during the serial transfer of an address byte, a data byte,

or an acknowledge bit. When a bus error occurs, TWINT is set. To recover from a bus error, the

TWSTO Flag must set and TWINT must be cleared by writing a logic one to it. This causes the

TWI to enter the not addressed slave mode and to clear the TWSTO Flag (no other bits in

TWCR are affected). The SDA and SCL lines are released, and no STOP condition is

transmitted.

20.7.6

Combining Several TWI Modes

In some cases, several TWI modes must be combined in order to complete the desired action.

Consider for example reading data from a serial EEPROM. Typically, such a transfer involves

the following steps:

The transfer must be initiated

The EEPROM must be instructed what location should be read

The reading must be performed

SLA

DATA

$A8

$B8

$B0

Reception of the own

slave address and one or

more data bytes

Last data byte transmitted.

Switched to not addressed

slave (TWEA = '0')

Arbitration lost as master

and addressed as slave

From master to slave

From slave to master

Any number of data bytes

and their associated acknowledge bits

This number (contained in TWSR) corresponds

to a defined state of the Two-wire Serial Bus. The

prescaler bits are zero or masked to zero

P or S

DATA

$C0

DATA

$C8

P or S

All 1's

Table 20-6.

Miscellaneous States

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR

To TWCR

STA

STO

TWINT

TWEA

$F8

No relevant state information

available; TWINT = “0”

No TWDR action

No TWCR action

Wait or proceed current transfer

$00

Bus error due to an illegal

START or STOP condition

No TWDR action

Only the internal hardware is affected, no STOP condi-

tion is sent on the bus. In all cases, the bus is released

and TWSTO is cleared.

ATmega32A Datasheet(PDF) 199 Page - ATMEL Corporation

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