PIC16 datasheet(184/418 Pages) MICROCHIP

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PIC16(L)F1508/9

DS40001609C-page 184

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21.2

SPI Mode Overview

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)

Figure 21-1 shows the block diagram of the MSSP

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 con-

nection is required from the master device to each

slave device.

Figure 21-4 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.

Figure 21-5 shows a typical connection between two

processors configured as master and slave devices.

Data is shifted out of both shift registers on the pro-

grammed 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 trans-

mits 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 polar-

ity.

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

reading this bit and saving it as the LSb of its shift

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 use-

ful 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 dese-

lects the slave.

Every slave device connected to the bus that has not

been selected through its slave select line must disre-

gard the clock and transmission signals and must not

transmit out any data of its own.

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