Serial Communication Buses: I2C and SPI

By Brody Dunn

Goals

Revisit Readings of I2C and SPI Not Insult You Realize Level of Understanding Take Next Step

Implementation

Inter-Integrated Circuit (I2C)

2-Wire Serial Communication Bus Introduced By Philips In 1992 Communication Protocol Between

Microcontroller And Peripherals Real Time Clocks (RTCs) Analog to Digital Converters (ADCs) Various Sensors Many, many, more

I2C Characteristics

Only 2 Wires Needed On Bus Serial Data (SDA) Serial Clock (SCL)

Multiple Devices Connected On Bus Typically Limited (i.e. 8, 20, 27, 210,...?)

Similar Code For All I2C Peripherals Supports Various Data Transfer Rates

Development of I2C

Version 1.0 Introduced in 1992 “Standard” 100 Kbits/s “Fast” 400 Kbits/s

Version 2.0 Released in 1998 “High-Speed” 3.4 Mbits/s

Version 2.1 Released in 2000 Clock Stretched “High-Speed” Timings Relaxed

How Does It Work?

Further Reading

I2C-Bus Specification Version 2.1 http://www.semiconductors.philips.com/acrobat_download/literature/9398/39340011.pdf

DS1631 Datasheet http://pdfserv.maxim-ic.com/en/ds/DS1631-DS1731.pdf

Serial Peripheral Interface (SPI)

Synchronous Communication Protocol 3-Wire (Plus 1 Chip-Select Pin/Device) Hardware On Microcontroller

Developed By Motorola “Loose” Standard

Broad Range of Devices Supported Memory (i.e. EEPROM, RAM, Etc.) Sensors

SPI Characteristics

3 Wires (Plus Chip Select Pin/Device) Master Out Slave In (MOSI) Master In Slave Out (MISO) Master/Slave Clock Output/Input (SCK) Chip/Slave Select (SS)

Multiple Devices Connected On Bus Limited Only By Number Of uC Pins

Supports Various Data Transfer Rates

How Does It Work?

Configurations (AT89S53)

SPI Control Register (SPCR) Sets Modes of Operation

SPI Status Register (SPSR) Checks Flags

SPI Data Register (SPDR) 8-Bit Shift Register Storing Data

4 Modes of SPI

Set In SPI Control Register (SPCR) Clock Polarity

Active Low Active High

Clock Phase Data Shifted On Rising Edge Data Shifted On Falling Edge

4 Modes of SPI (CPHA = 0)

4 Modes of SPI (CPHA = 1)

Further Reading

AT89S53 Datasheet (Pg. 8, 14-16) http://www.atmel.com/dyn/resources/prod_documents/DOC0787.PDF

Pont Ch. 24

SPI Summary

3-Wire (+ SS/Device) 1.5 MHz Bit Frequency (AT89S53) Microcontroller Hardware Supports Many Devices