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2545M–AVR–09/07
20. USART in SPI Mode
20.1 Features
• Full Duplex, Three-wire Synchronous Data Transfer
• Master Operation
• Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3)
• LSB First or MSB First Data Transfer (Configurable Data Order)
• Queued Operation (Double Buffered)
• High Resolution Baud Rate Generator
• High Speed Operation (fXCKmax = fCK/2)
• Flexible Interrupt Generation
20.2 Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be
set to a master SPI compliant mode of operation. Setting both UMSELn1:0 bits to one enables
the USART in Master SPI Mode (MSPIM) logic. In this mode of operation the SPI master control
logic takes direct control over the USART resources. These resources include the transmitter
and receiver shift register and buffers, and the baud rate generator. The parity generator and
checker, the data and clock recovery logic, and the RX and TX control logic is disabled. The
USART RX and TX control logic is replaced by a common SPI transfer control logic. However,
the pin control logic and interrupt generation logic is identical in both modes of operation.
The I/O register locations are the same in both modes. However, some of the functionality of the
control registers changes when using MSPIM.
20.3 Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. For
USART MSPIM mode of operation only internal clock generation (i.e. master operation) is sup-
ported. The Data Direction Register for the XCKn pin (DDR_XCKn) must therefore be set to one
(i.e. as output) for the USART in MSPIM to operate correctly. Preferably the DDR_XCKn should
be set up before the USART in MSPIM is enabled (i.e. TXENn and RXENn bit set to one).
The internal clock generation used in MSPIM mode is identical to the USART synchronous mas-
ter mode. The baud rate or UBRRn setting can therefore be calculated using the same
equations, see Table 20-1:
Table 20-1. Equations for Calculating Baud Rate Register Setting
Operating Mode
Equation for Calculating Baud
Rate
(1)
Equation for Calculating UBRRn
Value
Synchronous Master
mode
BAUD
f
OSC
2 UBRRn 1+()
-------------------------------------- -= UBRRn
f
OSC
2BAUD
-------------------- 1–=
