ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 202

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Overview of the TWI
Module
SCL and SDA Pins
Bit Rate Generator Unit
202
ATmega64(L)
The TWI module is comprised of several submodules, as shown in Figure 94. All regis-
ters drawn in a thick line are accessible through the AVR data bus.
Figure 94. Overview of the TWI Module
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 pull-ups 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.
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:
•
TWBR = Value of the TWI Bit Rate Register.
Slew-rate
Address Match Unit
Arbitration Detection
Control
START / STOP
Address Comparator
Address Register
Control
SCL
(TWAR)
Spike
Filter
Bus Interface Unit
SCL frequency
Spike Suppression
Address/Data Shift
Register (TWDR)
Slew-rate
Control
SDA
=
Status Register
---------------------------------------------------------- -
16
CPU Clock frequency
Ack
Spike
Filter
(TWSR)
+
2(TWBR) 4
State Machine and
Control Unit
Status Control
⋅
TWPS
Control Register
Bit Rate Generator
(TWCR)
Bit Rate Register
Prescaler
(TWBR)
2490G–AVR–03/04