174
AT90PWM216/316 [DATASHEET]
7710H–AVR–07/2013
Figure 17-1. USART Block Diagram
(1)
Note:
The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top): Clock
Generator, Transmitter and Receiver. Control registers are shared by all units. The Clock Generation logic consists
of synchronization logic for external clock input used by synchronous slave operation, and the baud rate generator.
The XCK (Transfer Clock) pin is only used by synchronous transfer mode. The Transmitter consists of a single
write buffer, a serial Shift Register, Parity Generator and Control logic for handling different serial frame formats.
The write buffer allows a continuous transfer of data without any delay between frames. The Receiver is the most
complex part of the USART module due to its clock and data recovery units. The recovery units are used for asyn-
chronous data reception. In addition to the recovery units, the Receiver includes a Parity Checker, Control logic, a
Shift Register and a two level receive buffer (UDR). The Receiver supports the same frame formats as the Trans-
mitter, and can detect Frame Error, Data OverRun and Parity Errors.
17.3
Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. The USART supports four
modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master synchronous and Slave
synchronous mode. The UMSEL bit in USART Control and Status Register C (UCSRC) selects between asynchro-
nous and synchronous operation. Double Speed (asynchronous mode only) is controlled by the U2X found in the
UCSRA Register. When using synchronous mode (UMSEL = 1), the Data Direction Register for the XCK pin
PARITY
GENERATOR
UBRR[H:L]
UDR (Transmit)
UCSRA
UCSRB
UCSRC
BAUD RATE GENERATOR
TRANSMIT SHIFT REGISTER
RECEIVE SHIFT REGISTER
RxD
TxD
PIN
CONTROL
UDR
(Receive)
PIN
CONTROL
XCK
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DATA
BUS
CLKio
SYNC LOGIC
Clock Generator
Transmitter
Receiver