Serial Peripheral Interface

By:- RishuMaster Slave

SCLKMOSIMISOSS

Index Introduction Overview Communication Advantages Disadvantages Applications

Introduction Communication Protocol Developed By Motorola Four Wire Protocol Serial Interface Master-Slave Approach Synchronous- Data clocked with Clock Signal Data Rate-10mbps Full Duplex Protocol Low power than I2C (no need of Pull ups) Supports Single master and multiple slaves No hardware slave acknowledgement

Overview SPI stands for Serial Peripheral Interface Used for moving data simply and quickly from one device to another Used to communicate across small distances Master-Slave(Multiple Slaves, Single Master) Serial Interface Synchronous Data Exchange

Continue… SPI is a Master-Slave protocol

The Master device controls the clock (SCK) No data is transferred unless a clock signal is present All slaves are controlled by the master clock The slave devices may not manipulate the clock Master Set Slave Select low Master Generates Clock Shift registers shift in and out data

Master Slave

SCLKMOSIMISOSS

Continue… Simple SPI Protocol Specifies 4 Signal Wires

Master Out Slave In (MOSI)

Master In Slave Out (MISO)

Serial Clock (SCLK)

Slave Select (SS)

MOSI – Carries data out of Master to Slave MISO – Carries data from Slave to Master (Both signals happen for every transmission) SS – Unique line to select a slave SCLK – Master produced clock to synchronize data transfer MOSI

Master Slave

SCLKMOSIMISOSS

Continue… SPI is a Synchronous protocol

The data is clocked along with a clock signal(SCK)

The clock signal controls when data is changed and when it should be read

Since SPI is synchronous, the clock rate can vary, unlike RS-232 style communications

Continue… SPI is a Data Exchange protocol

As data is being clocked out, new data is clocked in

Data is exchanged - no device can just be a transmitter only or receiver only

the master controls the exchange by manipulating the clock line (SCK)

Often a signal controls when a device is accessed - this is the CS or SS signal

CS or SS signal is known as “Chip Select” or “Slave Select” and is frequently an active-low signal.

Data is only output during the rising or falling edge of SCK Data is latched during the opposite edge of SCK The opposite edge is used to ensure data is valid at the time of reading

Fig.1 : Two SPI Modules connected in a Master-Slave Configuration

Master shifts out data to Slave, and shift in data from Slave

Serial Peripheral Interface(SPI)

Data is shifted out of the master's MOSI pin and in its MISO pin.Data transfer is initiated by simply writing data to the SPI data register. All data movement is coordinated by SCK. Slave select may or may not be used depending on interfacing device. To get input data only you send “junk” data to SPDR to start the clock.

Single Master and Single Slave

Master Slave

SCLKMOSIMISOSS

Master• Initiates the Connection• Controls SCLK and Data transfer Slave• Transmits Data• Receives Data

Single Master and Multiple independent Slave

Master

SCLKMOSIMISOSS1SS2

SCLKMOSIMISO

SS

SCLKMOSIMISOSS

Master and multiple daisy-chained slaves

How Do They Communicate Communication Initiated by Master only Master Configures the clock – Frequency less than equal to maximum frequency Slave

Support Master Selects Slave – By Pulling chip select(SS) of particular Slave-peripheral to Low State

Data Transmission Mode

SPI Mode CPOL CPHA Shift SCK Edge

Capture SCK Edge

0 0 0 Falling Raising1 0 1 Raising Falling2 1 0 Raising Falling3 1 1 Falling Raising

Table : SPI Mode Configuration

SPI Transfer format with CPHA = 0

SPI Transfer format with CPHA = 1

Steps for Master Mode Set SPCCR (Clock Counter Register) with appropriate clock frequency Set SPCR (Control Register) to desired settings Write the data into SPDR (Data Register) of SPI Monitor SPIF bit of SPCR register, until it sets to 1(SPIF = 1 means data transfer is complete ). Read SPSR (Status Register), this will clear SPIF bit Read SPDR (Data Register), reading data register will return data sent by the slave during last

transfer.

Steps for Slave Mode Clock will be decided by Master Set SPCR (Control Register) to desired settings Write dummy data into SPDR (Data Register), so that SPI control block will generate Clock. Monitor SPIF bit of SPCR register, until it sets to 1 Read SPSR (Status Register), this will clear SPIF bit Read SPDR (Data Register), reading data register will return data sent by the slave during last

transfer.

SPI Clock Counter Register (SPCCR) The SPCCR is used to set data transfer rate (SPI Frequency) It is 8 bit register, and the value

entered in this register is used to calculate frequency

SPI data rate = (PCLK / SPCCR value)

The value entered must be even value, thus LSB must be 0.The value should be greater than 8.So maximum frequency is 1.875 MHz(PCLK = 15MHz).

SPI Control Register (SPCR)

SPIE SPE LSBF MSTR CPOL CPHA SPR1 SPR0

Bit 7 6 5 4 3 2 1 0

Read/Write

Initial Value

R/W R/W R/W R/W R/W R/W R/W R/W

0 0 0 0 0 0 0 0

SPCR

SPI Control Register (SPCR)Bit 0&1: SPR0& SPR1

SPI2X

SPR1 SPR0 SCLK

0 0 0 fosc/40 0 1 fosc/160 1 0 fosc/640 1 1 fosc/1281 0 0 fosc/21 0 1 fosc/81 1 0 fosc/321 1 1 fosc/64

In SPSR Clock Rate

SPI Control Register (SPCR)Bit 2: CPHA

CPHA stands for Clock Phase Control, and plays an important role in deciding the relation between sampling of data and clock pulse

CPHA = 0 ; data is sampled on first clock edge CPHA = 1 ; data is sampled on the second rising edge

•The important thing to be considered here is CPHA should be 0 when LPC2148 is used as a SPI Master. •It is mentioned in the User Manual of LPC2148 that SSEL signal is inactive during the data transfer when CPHA is 0, but when CPHA is 1 the SSEL signal becomes active and immediately transforms itself into slave.•This results into a Mode Fault and data transfer terminates. •In this case MODF bit of Status Register will set to 1.

SPI Control Register (SPCR)

Bit 3 : CPOL

•CPOL stands for Clock Polarity Control•The bit decides the polarity of SPI clock, when set to 1 clock is active low. In that casefirst clock will start with negative going pulse•when set to 0 the clock is active high meaning that the clock will start with positive going edge

CPOL = 0

CPOL = 1

SPI Control Register (SPCR)Bit 4 : MSTR

The bit is used to configure SPI block in Master/Slave Mode

MSTR = 0 Slave ModeMSTR = 1 Master Mode

Bit 5 : DORD or LSBF

The bit decides the direction of bit transfer

LSBF = 0 MSB is transferred first LSBF = 1 LSB is transferred first

SPI Control Register (SPCR)Bit 6 : SPE

•The bit is used to enable SPI InterfaceSPE = 0 disable SPI InterfaceSPE = 1 enable SPI Interface

•It is an interrupt Enable bit,

SPIE = 0 Interrupt are disabled SPIE = 1 Interrupts are enabled and will occur when SPI/ WCOL bit is set.

Bit 7 : SPIE

SPI Status Register (SPSR)Bit 0 : SPI2X

Bit 6 : WCOL

•The bit is used to set double clock rate in master mode

SPI2X = 0 normal clock rateSPI2X = 1 double clock rate

•The bit is used for write collision

WCOL is set if SPDR is written during a receive transfer

SPI Status Register (SPSR)Bit 7 : SPIF

•The bit is used for interrupt flag

SPIF bit is set when serial transfer is complete.

SPIF WCOL - - - - - SPI2X SPSR

Bit 7 6 5 4 3 2 1 0

Read/Write

Initial Value

R R R R R R R R/W

0 0 0 0 0 0 0 0

SPI Data Register (SPDR) It is 16 bit register used in data transfer, the data length is selectable (8 –16bits). The data length can be configured by using bit 2 (BitEnable) and bits 11:8 of control register. There is no buffer between the data register and the internal shift register. A write to the data register goes directly into the internal shift register. Therefore, data should only be written to this register when a transmit is not currently in

progress. Otherwise a Collision Error may occur. Read data is buffered. When a transfer is complete, the receive data is transferred to a single

byte data buffer, where it is later read.

MSB - - - - - - LSB SPDR

Bit 7 6 5 4 3 2 1 0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/WX X X X X X X X

Advantages Full Duplex Communication Higher Throughput than I2C Not Limited to 8 bit words in case of bit transferring Arbitrary choice of message size, content and Purpose Low Power Fast and easy

Fast for point-to-point connections Easily allows streaming/Constant data inflow No addressing/Simple to implement

Everyone supports it

Disadvantages Requires more pins than I2C No hardware flow control No Slave Acknowledgement ability No inherent arbitration Multi Master Difficult to Implement SS makes multiple slaves very complicated Short Distance

SPI Peripherals Converters (ADC, DAC) Memories (EEPROM, RAM’s, Flash) Sensors (Temperature, Humidity, Pressure) Real Time Clocks Misc.- Potentiometers, LCD controllers, UART’s, USB controller, CAN controller, amplifiers

Some Important Facts A read or write of the SPI data register is required in order to clear the SPIF

status bit. The prime function of SPSR register is to indicate the completion of data

transfer between two devices. The remaining bits of SPSR register. When CPHA = 0, the SSEL signal will always go inactive between data

transfers. This is not guaranteed when CPHA = 1 (the signal can remain active).

Thank You !!!