125/317
5-Peripherals
5.7 SERIAL PERIPHERAL INTERFACE
The Serial Peripheral Interface is a device that allows synchronous, bit serial communication
between a microcontroller and a peripheral such as a serial-access memory, a Liquid Crystal
Display module, etc.; or between two or more microcontrollers. This interface only requires
three lines, thus saving pins for other purposes. The data is sent on a byte by byte basis. To
perform the same function using a byte-wide parallel interface, at least ten lines are needed:
eight for the data plus two for the handshake.
When connected to a peripheral, the SPI is configured as a master; when connected to an-
other microcontroller, it is configured either as a master or as a slave.
The maximum bit rate of a master is 4 MHz with a 16-MHz crystal, or 250 kBytes per second.
With this speed, serial transmission is just as fast as parallel transmission, since the instruc-
tions needed to put or get a byte through the channel take longer than the transmission.
Actually, transfer is bi-directional; at the same time that data is sent on one line, it is received
from the other line; this allows full-duplex transmission at the rate mentioned above when two
microcontrollers are connected together.
A supplementary pin, SS
, allows the SPI to be enabled when it is in slave mode. This allows
a master to be connected to several slaves and to communicate with only one slave at a time.
All the logic needed for multidrop communication is provided, including collision detection.
This permits reliable interprocessor communication. When the SPI is in master mode, this pin
must be set to a high level.
Here is an example of two microcontrollers connected together:
05-spi1
MOSI
MISO
MOSI
MISO
SCK
SCK
SS
SS
V
DD
permanent
MASTER
configuration
8-bit shift register
SPI Clock
generator
8-bit shift register
permanent
SLAVE
configuration
Simplest configuration for a dialogue with SPIs
between two microcontrollers
MSTR=1
MSTR=0
To Next Page
Loading...
