Rainbow Electronics ATmega128RFA1 - Manual (Page 388)

388
8266A-MCU Wireless-12/09
ATmega128RFA1
details of the following serial transfer are given in Table 25-3 on page 390 to Table 25-6
on page 398. Note that the prescaler bits are masked to zero in these tables.
25.7.1 Master Transmitter Mode
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave
Receiver (see
Figure 25-11 below ). In order to enter a Master mode, a START
condition must be transmitted. The format of the following address packet determines
whether Master Transmitter or Master Receiver mode is to be entered. If SLA+W is
transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All
status codes mentioned in this section assume that the prescaler bits are zero or are
masked to zero.
Figure 25-11. Data Transfer in Master Transmitter Mode
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3 Device n
SDA
SCL
........ R1 R2
DEVDD
A START condition is sent by writing the following value to TWCR:
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1 X 1 0 X 1 0 X
TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one
to transmit a START condition and TWINT must be written to one to clear the TWINT
Flag. The TWI will then test the 2-wire Serial Bus and generate a START condition as
soon as the bus becomes free. After a START condition has been transmitted, the
TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (see
Table
25-3 on page 390). In order to enter MT mode, SLA+W must be transmitted. This is
done by writing SLA+W to TWDR. Thereafter the TWINT bit should be cleared (by
writing it to one) to continue the transfer. This is accomplished by writing the following
value to TWCR:
TWCR
TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value
1 X 0 0 X 1 0 X
When SLA+W have been transmitted and an acknowledgement bit has been received,
TWINT is set again and a number of status codes in TWSR are possible. Possible
status codes in Master mode are 0x18, 0x20, or 0x38. The appropriate action to be
taken for each of these status codes is detailed in
Table 25-3 on page 390.
When SLA+W has been successfully transmitted, a data packet should be transmitted.
This is done by writing the data byte to TWDR. TWDR must only be written when
TWINT is high. If not, the access will be discarded, and the Write Collision bit (TWWC)
will be set in the TWCR Register. After updating TWDR, the TWINT bit should be
cleared (by writing it to one) to continue the transfer. This is accomplished by writing the
following value to TWCR: