MODULE -12
SPI Bus interface
Introduction:
Serial to Peripheral Interface (SPI) is a hardware/firmware
communications protocol developed by Motorola and later
adopted by others in the industry. Microwire of National
Semiconductor is same as SPI. Sometimes SPI is also called
a "four wire" serial bus.
The Serial Peripheral Interface or SPI-bus is a simple
4-wire serial communications interface used by many microprocessor/microcontroller
peripheral chips that enables the controllers and peripheral
devices to communicate each other. Even though it is developed
primarily for the communication between host processor and
peripherals, a connection of two processors via SPI is just
as well possible.
The SPI bus, which operates at full duplex (means, signals
carrying data can go in both directions simultaneously),
is a synchronous type data link setup with a Master / Slave
interface and can support up to 1 megabaud or 10Mbps of
speed. Both single-master and multi-master protocols are
possible in SPI. But the multi-master bus is rarely used
and look awkward, and are usually limited to a single slave.
The SPI Bus is usually used only on the PCB. There are
many facts, which prevent us from using it outside the PCB
area. The SPI Bus was designed to transfer data between
various IC chips, at very high speeds. Due to this high-speed
aspect, the bus lines cannot be too long, because their
reactance increases too much, and the Bus becomes unusable.
However, its possible to use the SPI Bus outside the PCB
at low speeds, but this is not quite practical.
The peripherals can be a Real Time Clocks, converters like
ADC and DAC, memory modules like EEPROM and FLASH, sensors
like temperature sensors and pressure sensors, or some other
devices like signal-mixer, potentiometer, LCD controller,
UART, CAN controller, USB controller and amplifier.
Data and control lines of the SPI and the basic connection:
An SPI protocol specifies 4 signal wires.
1. Master Out Slave In (MOSI) - MOSI signal is generated
by Master, recipient is the Slave.
2. Master In Slave Out (MISO) - Slaves generate MISO signals
and recipient is the Master.
3. Serial Clock (SCLK or SCK) - SCLK signal is generated
by the Master to synchronize data transfers between
the master and the slave.
4. Slave Select (SS) from master to Chip Select (CS) of
slave - SS signal is generated by Master to select
individual slave/peripheral devices. The SS/CS is an active
low signal.
There may be other naming conventions such as Serial Data
In [SDI] in place of MOSI and Serial Data Out [SDO] for
MISO.
Among these four logic signals, two of them MOSI &
MISO can be grouped as data lines and other two SS &
SCLK as control lines.
As we already know, in SPI-bus communication there can
be one master with multiple slaves. In single-master protocol,
usually one SPI device acts as the SPI Master and controls
the data flow by generating the clock signal (SCLK) and
activating the slave it wants to communicate with slave-select
signal (SS), then receives and or transmits data via the
two data lines. A master, usually the host micro controller,
always provides clock signal to all devices on a bus whether
it is selected or not.
The usage of these each four pins may depend on the devices.
For example, SDI pin may not be present if a device does
not require an input (ADC for example), or SDO pin may not
be present if a device does not require an output (LCD controllers
for example). If a microcontroller only needs to talk to
1 SPI Peripheral or one slave, then the CS pin on that slave
may be grounded. With multiple slave devices, an independent
SS signal is needed from the master for each slave device.
How do they communicate:
The communication is initiated by the master all the time.
The master first configures the clock, using a frequency,
which is less than or equal to the maximum frequency that
the slave device supports. The master then select the desired
slave for communication by pulling the chip select (SS)
line of that particular slave-peripheral to "low"
state. If a waiting period is required (such as for analog-to-digital
conversion) then the master must wait for at least that
period of time before starting to issue clock cycles.
The slaves on the bus that has not been activated by the
master using its slave select signal will disregard the
input clock and MOSI signals from the master, and must not
drive MISO. That means the master selects only one slave
at a time.
Most devices/peripherals have tri-state outputs, which
goes to high impedance state (disconnected) when the device
is not selected. Devices without this tri-state outputs
cannot share SPI bus with other devices, because such slave's
chip-select may not get activated.
A full duplex data transmission can occur during each clock
cycle. That means the master sends a bit on the MOSI line;
the slave reads it from that same line and the slave sends
a bit on the MISO line; the master reads it from that same
line.
Data transfer is organized by using Shift register with
some given word size such as 8- bits (remember, its not
limited to 8-bits), in both master and slave. They are connected
in a ring. While master shifts register value out through
MOSI line, the slave shifts data in to its shift register.
Data are usually shifted out with the MSB first, while
shifting a new LSB into the same register. After that register
has been shifted out, the master and slave have exchanged
their register values. Then each device takes that value
and does the necessary operation with it (for example, writing
it to memory). If there are more data to be exchanged, the
shift registers are loaded with new data and the process
is repeated. When there are no more data to be transmitted,
the master stops its clock. Normally, it then rejects the
slave.
There is a "multiple byte stream mode" available
with SPI bus interface. In this mode the master can shift
bytes continuously. In this case, the slave select (SS)
is kept low until all stream process gets finished.
SPI devices sometimes use another signal line to send an
interrupt signal to a host CPU. Some of the examples for
these type of signals are pen-down interrupts from touch-screen
sensors, thermal limit alerts from temperature sensors,
alarms issued by real time clock chips, and headset jack
insertions from the sound codec in a cell phone.
Significance of the clock polarity and phase:
Another pair of parameters called clock polarity (CPOL)
and clock phase (CPHA) determine the edges of the clock
signal on which the data are driven and sampled.
That means, in addition to setting the clock frequency,
the master must also configure the clock polarity (CPOL)
and phase (CPHA) with respect to the data. Since the clock
serves as synchronization of the data communication, there
are four possible modes that can be used in an SPI protocol,
based on this CPOL and CPHA.
| SPI Mode |
CPOL |
CPHA |
| 0 |
0 |
0 |
| 1 |
0 |
1 |
| 2 |
1 |
0 |
| 3 |
1 |
1 |
If the phase of the clock is zero (i.e. CPHA = 0) data
is latched at the rising edge of the clock with CPOL = 0,
and at the falling edge of the clock with CPOL = 1.
If CPHA = 1, the polarities are reversed. Data is latched
at the falling edge of the clock with CPOL = 0, and at the
rising edge with CPOL = 1.
The micro-controllers allow the polarity and the phase
of the clock to be adjusted. A positive polarity results
in latching data at the rising edge of the clock. However
data is put on the data line already at the falling edge
in order to stabilize. Most peripherals, which can only
be slaves, work with this configuration. If it should become
necessary to use the other polarity, transitions are reversed.
Different types of configurations:
Suppose a master-microcontroller needs to talk to multiple
SPI Peripherals. There are 2 ways to set things up:
1. Cascaded slaves or daisy-chained slaves
2. Independent slaves or parallel configuration
Daisy-chained slave configuration:
In cascaded slave configuration, all the clock lines (SCLK)
are connected together. And also all the chip select (CS)
pins are connected together. The data flows out the microcontroller,
through each peripheral in turn, and back to the microcontroller.
The data output of the preceding slave-device is tied to
the data input of the next, thus forming a wider shift register.
So the cascaded slave-devices are evidently looked at as
one larger device and receive therefore the same chip select
signal. This means, only a single SS line is required from
the master, rather than a separate SS line for each slave.
But we have to remember that the daisy-chain will not work
with devices which support or require multiple bytes operation.
Independent slave configuration:
This is the typical SPI-bus configuration with one SPI-master
and multiple slaves/peripherals. In this independent or
parallel slave configuration,
1. All the clock lines (SCLK) are connected together.
2. All the MISO data lines are connected together.
3. All the MOSI data lines are connected together.
4. But the Chip Select (CS) pin from each peripheral must
be connected to a separate Slave Select (SS) pin
on the master-microcontroller.
Queued Serial Peripheral Interface (QSPI)
The queued serial peripheral interface (QSPI) is another
type of SPI controller, not another bus type. Or in other
words it is just an extension to the SPI-bus.
The difference is that it uses a data queue with programmable
queue pointers that allow some data transfers without CPU
intervention. It also has a wrap-around mode that allows
continuous transfers to and from the queue with no CPU intervention.
As a result, the peripherals or the slaves appear to the
CPU as memory-mapped parallel devices. This feature is useful
in applications such as control of an Analog to Digital
converter.
The QSPI has got some more programmable features like chip
selects and transfer length/delay.
Advantages of SPI
1. Full duplex communication
2. Higher throughput than I²C protocol
3. Not limited to 8-bit words in the case of bit-transferring
4. Arbitrary choice of message size, contents, and purpose
5. Simple hardware interfacing
6. Typically lower power requirements than I²C due
to less circuitry.
7. No arbitration or associated failure modes.
8. Slaves use the master's clock, and don't need precision
oscillators.
9. Transceivers are not needed.
10. At most one "unique" bus signal per device
(CS); all others are shared
Disadvantages of SPI
1. Requires more pins on IC packages than I²C
2. No in-band addressing. Out-of-band chip select signals
are required on shared busses.
3. No hardware flow control
4. No slave acknowledgment
5. Multi-master busses are rare and awkward, and are usually
limited to a single slave.
6. Without a formal standard, validating conformance is
not possible
7. Only handles short distances compared to RS-232, RS-485,
or CAN.
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module - 12 (I2C Interface)
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