Datasheet

214
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
19. USART in SPI Mode
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be set to a master
SPI compliant mode of operation. The Master SPI Mode (MSPIM) has the following 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 (f
XCK
max = f
CK
/2)
• Flexible Interrupt Generation
19.1 Overview
Setting both UMSELn1:0 bits to one enables the USART in 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.
19.2 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 supported. 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 master mode. The
baud rate or UBRRn setting can therefore be calculated using the same equations in the following table:
Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps).
BAUD: Baud rate (in bits per second, bps).
f
OSC
: System Oscillator clock frequency.
UBRRn: Contents of the UBRRnH and UBRRnL Registers, (0-4095).
19.3 SPI Data Modes and Timing
There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which are
determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are shown in Figure 19-1.
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–=