Microchip Technology AC244045 Data Sheet

DS41440C-page 244

 2010-2012 Microchip Technology Inc.

The Serial Peripheral Interface (SPI) bus is a
synchronous serial data communication bus that
operates in Full Duplex mode. Devices communicate in
a master/slave environment where the master device
initiates the communication. A slave device is
controlled through a chip select known as Slave Select. 
The SPI bus specifies four signal connections:
• Serial Clock (SCKx)
• Serial Data Out (SDOx)
• Serial Data In (SDIx)
• Slave Select (SSx)

 shows the block diagram of the MSSPx

module when operating in SPI Mode.
The SPI bus operates with a single master device and
one or more slave devices. When multiple slave
devices are used, an independent Slave Select
connection is required from the master device to each
slave device.

 shows a typical connection between a

master device and multiple slave devices.
The master selects only one slave at a time. Most slave
devices have tri-state outputs so their output signal
appears disconnected from the bus when they are not
selected.
Transmissions involve two shift registers, eight bits in
size, one in the master and one in the slave. With either
the master or the slave device, data is always shifted
out one bit at a time, with the Most Significant bit (MSb)
shifted out first. At the same time, a new Least
Significant bit (LSb) is shifted into the same register.

 shows a typical connection between two

processors configured as master and slave devices.
Data is shifted out of both shift registers on the
programmed clock edge and latched on the opposite
edge of the clock.
The master device transmits information out on its
SDOx output pin which is connected to, and received
by, the slave’s SDIx input pin. The slave device
transmits information out on its SDOx output pin, which
is connected to, and received by, the master’s SDIx
input pin.
To begin communication, the master device first sends
out the clock signal. Both the master and the slave
devices should be configured for the same clock
polarity. 
The master device starts a transmission by sending out
the MSb from its shift register. The slave device reads
this bit from that same line and saves it into the LSb
position of its shift register. 
During each SPI clock cycle, a full duplex data
transmission occurs. This means that while the master
device is sending out the MSb from its shift register (on

its SDOx pin) and the slave device is reading this bit
and saving it as the LSb of its shift register, that the
slave device is also sending out the MSb from its shift
register (on its SDOx pin) and the master device is
reading this bit and saving it as the LSb of its shift
register.
After eight bits have been shifted out, the master and
slave have exchanged register values.
If there is more data to exchange, the shift registers are
loaded with new data and the process repeats itself.
Whether the data is meaningful or not (dummy data),
depends on the application software. This leads to
three scenarios for data transmission:
• Master sends useful data and slave sends dummy 

data.

• Master sends useful data and slave sends useful 

data.

• Master sends dummy data and slave sends useful 

data.

Transmissions may involve any number of clock
cycles. When there is no more data to be transmitted,
the master stops sending the clock signal and it
deselects the slave.
Every slave device connected to the bus that has not
been selected through its slave select line must
disregard the clock and transmission signals and must
not transmit out any data of its own.