AT90SCR100 datasheet(308/433 Pages) ATMEL

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308

TPR0327AY–SMS–30Jan09

AT90SCR100

22.7

Transmission Modes

The TWI can operate in one of four major modes. These are named Master Transmitter (MT),

Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these

modes can be used in the same application. As an example, the TWI can use MT mode to write

data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters

are present in the system, some of these might transmit data to the TWI, and then SR mode

would be used. It is the application software that decides which modes are legal.

The following sections describe each of these modes. Possible status codes are described

along with figures detailing data transmission in each of the modes. These figures contain the

following abbreviations:

S: START condition

Rs: REPEATED START condition

R: Read bit (high level at SDA)

W: Write bit (low level at SDA)

A: Acknowledge bit (low level at SDA)

A: Not acknowledge bit (high level at SDA)

Data: 8-bit data byte

P: STOP condition

SLA: Slave Address

In Figure 22-12 to Figure 22-18, circles are used to indicate that the TWINT Flag is set. The

numbers in the circles show the status code held in TWSR, with the prescaler bits masked to

zero. At these points, actions must be taken by the application to continue or complete the TWI

transfer. The TWI transfer is suspended until the TWINT Flag is cleared by software.

When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate soft-

ware action. For each status code, the required software action and details of the following serial

transfer are given in Table 22-2 to Table 22-5. Note that the prescaler bits are masked to zero in

these tables.

wait3:

r16,TWCR

sbrs

r16,TWINT

rjmp

wait3

while

(!(TWCR & (1<<TWINT)))

;

Wait for TWINT Flag set. This

indicates that the DATA has

been transmitted, and

ACK/NACK has been received.

r16,TWSR

andi

r16, 0xF8

cpi

r16, MT_DATA_ACK

brne

ERROR

((TWSR & 0xF8) !=

MT_DATA_ACK)

ERROR();

Check value of TWI Status

status different from

MT_DATA_ACK go to ERROR

ldi

r16,

(1<<TWINT)|(1<<TWEN)|

(1<<TWSTO)

out

TWCR, r16

TWCR = (1<<TWINT)|(1<<TWEN)|

(1<<TWSTO);

Transmit STOP condition

Assembly Code Example

C Example

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