ATMEGA640 datasheet(195/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

21.1 SS Pin Functionality

21.1.1

Slave Mode

When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is held low, the SPI is

activated, and MISO becomes an output if configured so by the user. All other pins are inputs. When SS is driven

high, all pins are inputs, and the SPI is passive, which means that it will not receive incoming data. Note that the

SPI logic will be reset once the SS pin is driven high.

The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the master

clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and receive logic,

and drop any partially received data in the Shift Register.

21.1.2

Master Mode

When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of the SS pin.

If SS is configured as an output, the pin is a general output pin which does not affect the SPI system. Typically, the

pin will be driving the SS pin of the SPI Slave.

If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin is driven low by

peripheral circuitry when the SPI is configured as a Master with the SS pin defined as an input, the SPI system

interprets this as another master selecting the SPI as a slave and starting to send data to it. To avoid bus conten-

tion, the SPI system takes the following actions:

1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of the SPI becoming a

Slave, the MOSI and SCK pins become inputs.

2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is set, the interrupt

routine will be executed.

Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that SS is

driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been cleared by a

slave select, it must be set by the user to re-enable SPI Master mode.

21.1.3

Data Modes

There are four combinations of SCK phase and polarity with respect to serial data, which are determined by control

bits CPHA and CPOL. The SPI data transfer formats are shown in Figure 21-3 on page 196 and Figure 21-4 on

page 196. Data bits are shifted out and latched in on opposite edges of the SCK signal, ensuring sufficient time for

data signals to stabilize. This is clearly seen by summarizing Table 21-3 on page 197 and Table 21-4 on page 197

in Table 21-2.

Table 21-2.

CPOL Functionality

Leading Edge

Trailing Edge

SPI Mode

CPOL=0, CPHA=0

Sample (Rising)

Setup (Falling)

CPOL=0, CPHA=1

Setup (Rising)

Sample (Falling)

CPOL=1, CPHA=0

Sample (Falling)

Setup (Rising)

CPOL=1, CPHA=1

Setup (Falling)

Sample (Rising)

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