ATmega32(L)
Overview of the TWI
Module
The TWI module is comprised of several submodules, as shown in Figure 84. All regis-
ters drawn in a thick line are accessible through the AVR data bus.
Figure 84. Overview of the TWI Module
SCL
SDA
Spike
Filter
Spike
Filter
Slew-rate
Control
Slew-rate
Control
Bus Interface Unit
Bit Rate Generator
START / STOP
Spike Suppression
Prescaler
Control
Address/Data Shift
Register (TWDR)
Bit Rate Register
(TWBR)
Arbitration detection
Ack
Address Match Unit
Control Unit
Address Register
(TWAR)
Status Register
(TWSR)
Control Register
(TWCR)
TWI Unit
State Machine and
Status control
Address Comparator
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 inter-
nal pullups in the AVR pads can be enabled by setting the PORT bits corresponding 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.
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 Pres-
caler 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 average TWI bus clock period. The SCL frequency is generated according
to the following equation:
CPU Clock frequency
SCL frequency = -----------------------------------------------------------
TWPS
16 + 2(TWBR) ⋅ 4
•
•
TWBR = Value of the TWI Bit Rate Register
TWPS = Value of the prescaler bits in the TWI Status Register
Note:
Pull-up resistor values should be selected according to the SCL frequency and the
capacitive bus line load. See Table 119 on page 290 for value of pull-up resistor.
175
2503J–AVR–10/06
ATMEL [ ATMEL ]
