Rainbow Electronics ATmega128RFA1 - Manual (Page 399)

399
8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 25-18. Formats and States in the Slave Transmitter Mode
S SLA R A DATA A
$A8 $B8
A
$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
n
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 A
A
$C8
P or S All 1’s
A
25.7.5 Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see Table
25-7 below .
Status 0xF8 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 0x00 indicates that a bus error has occurred during a 2-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.
Table 25-7. Miscellaneous States
Application Software Response
To TWCR
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire
Serial Bus and 2-wire
Serial Interface Hard-
ware To/from TWDR STA STO TWINT TWEA
Next Action Taken by TWI
Hardware
0xF8 No relevant state
information available TWDR action No TWCR action Wait or proceed current transfer
0x00 Bus error due to an illegal
START or STOP condition No TWDR action 0 1 1 X 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.
25.7.6 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:
1. The transfer must be initiated.
2. The EEPROM must be instructed what location should be read.