Rainbow Electronics ATmega128RFA1 - Manual (Page 386)

386
8266A-MCU Wireless-12/09
ATmega128RFA1
clears the flag. The TWI will not start any operation as long as the TWINT bit in
TWCR is set. Immediately after the application has cleared TWINT, the TWI will
initiate transmission of the data packet.
6. When the data packet has been transmitted, the TWINT Flag in TWCR is set, and
TWSR is updated with a status code indicating that the data packet has successfully
been sent. The status code will also reflect whether a Slave acknowledged the
packet or not.
7. The application software should now examine the value of TWSR, to make sure that
the data packet was successfully transmitted, and that the value of the ACK bit was
as expected. If TWSR indicates otherwise, the application software might take some
special action, like calling an error routine. Assuming that the status code is as
expected, the application must write a specific value to TWCR, instructing the TWI
hardware to transmit a STOP condition. Which value to write is described later on.
However, it is important that the TWINT bit is set in the value written. Writing a one
to TWINT clears the flag. The TWI will not start any operation as long as the TWINT
bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI
will initiate transmission of the STOP condition. Note that TWINT is NOT set after a
STOP condition has been sent.
Even though this example is simple, it shows the principles involved in all TWI
transmissions. These can be summarized as follows:
• When the TWI has finished an operation and expects application response, the
TWINT Flag is set. The SCL line is pulled low until TWINT is cleared.
• When the TWINT Flag is set, the user must update all TWI Registers with the value
relevant for the next TWI bus cycle. As an example, TWDR must be loaded with the
value to be transmitted in the next bus cycle.
• After all TWI Register updates and other pending application software tasks have
been completed, TWCR is written. When writing TWCR, the TWINT bit should be
set. Writing a one to TWINT clears the flag. The TWI will then commence executing
whatever operation was specified by the TWCR setting.
In the following an assembly and C implementation of the example is given. Note that
the code below assumes that several definitions have been made, for example by using
include-files.
Table 25-2. Code example
Assembly Code Example C Example Comments
1 ldi
r16,(1<<TWINT)|(1<<TWSTA)|
(1<<TWEN)
out TWCR, r16
TWCR = (1<<TWINT)|(1<<TWSTA)|
(1<<TWEN)
Send START condition
2 wait1:
in r16,TWCR
sbrs r16,TWINT
rjmp wait1
while (!(TWCR & (1<<TWINT))); Wait for TWINT Flag set. This
indicates that the START condition
has been transmitted
in r16,TWSR
andi r16, 0xF8
cpi r16, START
brne ERROR
if ((TWSR & 0xF8) != START)
ERROR();
Check value of TWI Status Register.
Mask prescaler bits. If status different
from START go to ERROR 3
ldi r16, SLA_W
out TWDR, r16
ldi r16, (1<<TWINT)|(1<<TWEN)
out TWCR, r16
TWDR = SLA_W;
TWCR = (1<<TWINT)|(1<<TWEN);
Load SLA_W into TWDR Register.
Clear TWINT bit in TWCR to start
transmission of address