Handy Tips
Also see the
following projects for more information/examples on SPI
Communication;
More and more serial bus systems are preferred instead of a parallel
bus, because of the simpler wiring. As the efficiency of serial
buses increases, the speed advantage of the parallel data
transmission gets less important. The clock frequencies of SPI
devices can go up to some Megahertz and more. There are a lot of
application where a serial transmission is perfectly sufficient.
The Principle
The
Serial Peripheral Interface is used primarily for a synchronous
serial communication of host processor and peripherals..
In the
standard configuration for a slave device (see illustration 1), two
control and two data lines are used. The data output SDO serves on
the one hand the reading back of data, offers however also the
possibility to cascade several devices. The data output of the
preceding device then forms the data input for the next IC.
Illustration 1: SPI
slave
There is
a MASTER and a SLAVE mode. The MASTER device provides the clock
signal and determines the state of the chip select lines, i.e. it
activates the SLAVE it wants to communicate with. CS and SCKL are
therefore outputs.
The
SLAVE device receives the clock and chip select from the MASTER, CS
and SCKL are therefore inputs.
This
means there is one master, while the number of slaves is only
limited by the number of chip selects.
A SPI
device can be a simple shift register up to an independent
subsystem. The basic principle of a shift register is always
present. Command codes as well as data values are serially
transferred, pumped into a shift register and are then internally
available for parallel processing. Here we already see an important
point, that must be considered in the philosophy of SPI bus systems:
The length of the shift registers is not fixed, but can differ from
device to device. Normally the shift registers are 8Bit or integral
multiples of it. Of course there also exist shift registers with an
odd number of bits. For example two cascaded 9Bit EEPROMs can store
18Bit data.
If a SPI
device is not selected, its data output goes into a high-impedance
state (hi-Z), so that it does not interfere with the currently
activated devices. When cascading several SPI devices, they are
treated as one slave and therefore connected to the same chip
select.
Thus
there are two meaningful types of connection of master and slave
devices. illustration 2 shows the type of connection for cascading
several devices.
Illustration 2:
Cascading several SPI devices
In
illustration 2 the cascaded devices are evidently looked at as one
larger device and receive therefore the same chip select. The data
output of the preceding device is tied to the data input of the
next, thus forming a wider shift register.
If
independent slaves are to be connected to a master an other bus
structure has to be chosen, as shown in illustration 3. Here, the
clock and the SDI data lines are brought to each slave. Also the SDO
data lines are tied together and led back to the master. Only the
chip selects are separately brought to each SPI device.
Illustration 3:
Master with independent slaves
Data
and Control Lines of the SPI
The SPI
requires two control lines (CS and SCLK) and two data lines (SDI and
SDO). The chip select line is named SS (Slave-Select).
With CS
(Chip-Select) the corresponding peripheral device is selected. This
pin is mostly active-low. In the unselected state the SDO lines are
hi-Z and therefore inactive. The master decides with which
peripheral device it wants to communicate. The clock line SCLK is
brought to the device whether it is selected or not. The clock
serves as synchronization of the data communication.
The
majority of SPI devices provide these four lines. Sometimes it
happens that SDI and SDO are multiplexed, for example in the
temperature sensor LM74 from National Semiconductor, or that one of
these lines is missing. A peripheral device which must or can not be
configured, requires no input line, only a data output. As soon as
it gets selected it starts sending data. In some ADCs therefore the
SDI line is missing (e.g.
MCP3001 from Microchip).
There
are also devices that have no data output. For example
LCD
controllers (e.g. COP472-3 from National Semiconductor), which can
be configured, but cannot send data or status messages.
 | Site Tutorial Index |
|  | 16F PIC Examples |
| | 18F PIC Examples |
|  | Handy Tips |
| | | High Power Switching |
| | | Serial Comms |