Datasheet

169
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
• A REPEATED START and a STOP condition
It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This implies
that in multi-master systems, all data transfers must use the same composition of SLA+R/W and data packets. In
other words: All transmissions must contain the same number of data packets, otherwise the result of the arbitra-
tion is undefined.
21.5 Overview of the TWI Module
The TWI module is comprised of several submodules, as shown in Figure 20-9. All registers drawn in a thick line
are accessible through the AVR data bus.
Figure 21-9. Overview of the TWI Module
21.5.1 SCL and SDA Pins
These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a slew-rate limiter in
order to conform to the TWI specification. The input stages contain a spike suppression unit removing spikes
shorter than 50 ns. Note that the internal pullups in the AVR pads can be enabled by setting the PORT bits corre-
sponding to the SCL and SDA pins, as explained in the I/O Port section. The internal pull-ups can in some systems
eliminate the need for external ones.
21.5.2 Bit Rate Generator Unit
This unit controls the period of SCL when operating in a Master mode. The SCL period is controlled by settings in
the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status Register (TWSR). Slave operation
does not depend on Bit Rate or Prescaler settings, but the CPU clock frequency in the slave must be at least 16
times higher than the SCL frequency. Note that slaves may prolong the SCL low period, thereby reducing the aver-
age TWI bus clock period. The SCL frequency is generated according to the following equation:
TWI Unit
Address Register
(TWAR)
Address Match Unit
Address Comparator
Control Unit
Control Register
(TWCR)
Status Register
(TWSR)
State Machine and
Status control
SCL
Slew-rate
Control
Spike
Filter
SDA
Slew-rate
Control
Spike
Filter
Bit Rate Generator
Bit Rate Register
(TWBR)
Prescaler
Bus Interface Unit
START / STOP
Control
Arbitration detection Ack
Spike Suppression
Address/Data Shift
Register (TWDR)
SCL frequency
CPU Clock frequency
16 2(TWBR) 4
TWPS
+
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