Low Speed Digital Interface Basics

Introduction

Inter-IC communication (IC-IC) has been established and evolved since the early days of electronic design. Several of these basic interfaces have stood the test of time and have been widely adopted across the electronics industry. We will examine a few basic and popular interfaces, such as I2C, SPI, and UART, that are used in almost any embedded system today.

Background

Digital communication largely fulfills two goals: sending instructions (i.e., control commands) and transferring data.

• Control commands initialize and configure devices for operation.

• Transferring data between on-board memory and peripheral devices is necessary for keeping an embedded system running as intended.

I/O Output Type

• Open-drain: An I/O output design that uses an external resistor to pull the interface to logic high and the internal NMOS transistor to drive the interface low.

• Push-pull: An I/O output design that uses a complementary pair of PMOS and NMOS transistors to drive the interface to logic level high or low.

Data Transfer Structure

• Serial: Data is sent and received one bit at a time in a sequential manner.

  • Minimal communication interface and logic are needed, but at the expense of lower data rate.

• Parallel: Data is sent and received in multiple bits at a time in a parallel manner.

  • Higher data rates can be achieved, but at the expense of a more complex communication interface and logic.

Data Transfer Technique

• Synchronous: Data is transmitted and received with a clock signal in a fixed time interval.

  • Source synchronous clocking: The transmitter device generates the clock needed for synchronous communication. This clocking technique is widely used in practice.

• Asynchronous: Data is transmitted and received without a clock in a random time interval.

Detailed Interface Descriptions

I2C

Main Purpose

I2C (Inter-Integrated Circuit) is a synchronous, half-duplex, serial communication bus that is commonly used for issuing instructions and establishing handshakes. It can also be used to transfer data, but it is typically used for slow output data devices such as sensors.

Main Features

• Supports multiple master and slave devices on one I2C bus.

• Half-duplex: Data can be transmitted or received, but not both at the same time.

• Slow data rate: Typical data rates are 100 kHz to 400 kHz.

• Open-drain output: The output drivers of I2C devices are open-drain, which means that they can only pull the bus line low. This allows multiple devices to share the same bus without conflict.

BUS Pins

• SDA (Serial Data): This pin is used to transmit and receive data.

• SCL (Serial Clock): This pin is used to synchronize the data transfer.

Protocol Basics

I2C communication is based on a simple protocol that consists of four types of messages:

• Start bit: This bit signals the beginning of a message.

• Slave address: This bit indicates the address of the slave device that the message is intended for.

• Command/data: This bit indicates whether the message is a command or data.

• Stop bit: This bit signals the end of a message.

7-bit slave addressing

I2C devices are addressed using a 7-bit address. This allows for 128 (2^7) unique slave addresses. However, in practice, it is not practical to have all 128 addresses on one bus due to limited bus bandwidth.

8-bit packet frame

Each I2C message consists of an 8-bit packet frame. The packet frame is structured as follows:

• Start bit (1 bit)

• Slave address (7 bits)

• R/W bit (1 bit)

• Data byte (8 bits)

• Stop bit (1 bit)

Arbitration

If two master devices attempt to access the bus at the same time, a process called arbitration is used to determine which device will have control of the bus. Arbitration is based on the principle of "first come, first served."

Slave clock stretching

If a slave device needs more time to process the current data before the next data frame is transferred, it can stretch the clock. Clock stretching is done by the slave device holding the clock line low until it is ready to continue.

Helpful Software Debug Tools

There are a number of software tools that can be used to debug I2C communication. One popular tool is the i2c-tools package, which is available for Linux. This package provides a number of commands that can be used to read and write data on the I2C bus, as well as to query the status of the bus.

Debug Hardware Tools

A I2C logic analyzer can be used to sniff, read, and write on I2C bus. Two popular logic analyzers are the Salae logic analyzer and the Aardvark I2C/SPI Host Adapter. These logic analyzers come with their own analysis software that can be used to view and analyze I2C traffic.

SPI

Main Purpose

SPI (Serial Peripheral Interface) is a synchronous, full-duplex, serial communication bus that is commonly used for transferring data. It is typically used for high-speed data transfer applications, such as connecting memory chips and flash drives.

Main Features

• Full-duplex: Data can be transmitted and received simultaneously.

• High data rate: Typical data rates are 20 MHz to 50 MHz.

• Push-pull output: The output drivers of SPI devices are push-pull, which means that they can both pull the bus line low and push it high. This allows only one driver per data line, which avoids bus contention.

BUS Pins

• MOSI (Master Out Slave In): This pin is used to transmit data from the master device to the slave device.

• MISO (Master In Slave Out): This pin is used to receive data from the slave device to the master device.

• CLK (Clock): This pin is used to synchronize the data transfer.

• CS (Chip Select): This pin is used to select the slave device that the master device is communicating with.

Debug Hardware Tool

A SPI logic analyzer can be used to sniff, read, and write on the SPI bus. Two popular logic analyzers are the Salae logic analyzer and the Aardvark I2C/SPI Host Adapter. These logic analyzers come with their own analysis software that can be used to view and analyze SPI traffic.