• TWIProtocol

OpenServo TWI/I2C Protocol

For the Version 3 development branch layout see TWIProtocol-dev

NOTE: I2C (pronounced "eye-square-see") is the two-wire bidirectional protocol utilized by the OpenServo for communication with a master controller. The protocol was invented by Philips and they have protected this name. For this reason other manufacturers have adopted different names for the same protocol. Atmel calls I2C "two wire interface" (TWI). This document adopts the TWI abbreviation used by Atmel for the bidirectional communication protocol to the OpenServo.

OpenServo Data Registers

The OpenServo implements 48 status registers that can be read from a master controller to modify and monitor the state of the servo. These registers are read using the TWI-bus DATA READ cycle and written using the TWI-bus DATA WRITE cycle.

NOTE: The OpenServo source code is the best documentation regarding the definition of each data register. Please refer to the 'registers.h' file for specific definitions of each register.

Read Only Registers

Registers 0x00 thru 0x0F are read-only and cannot be modified by the master controller. These registers implement servo status values and flags that can be modified by commands.

Read/Write Registers

Registers 0x10 thru 0x1F are always read-write enabled and can be modified by the master controller. These values implement dynamic servo control such as positioning.

Read/Write Protected Registers

Registers 0x20 thru 0x2F are write protected in that they can only be written by the master controller when the write enable flag is set via the WRITE_ENABLE command. These registers implement configuration information and are normally only written to during initial configuration of the servo.

OpenServo Register Table

OpenServo Commands

The OpenServo implements the following commands that can be sent by the master controller to enable certain operations of the servo. These commands are sent to the OpenServo using the TWI-bus COMMAND cycle.

NOTE: Please refer to the 'twi.h' file for specific definitions of each command.

OpenServo TWI-bus Protocol

Before any data is transmitted on the TWI-bus, the slave device that should respond is addressed first. Slave addressing is always carried out with the first byte transmitted after the start condition. The TWI-bus configuration for the COMMAND, DATA WRITE and DATA READ cycles is shown below.

TWI-bus COMMAND Cycle

After the slave address with the read/write flag set to 0, an unlimited number of command bytes can be sent. Each command byte must have the address/command flag (most significant bit) set to 1. A DATA WRITE/READ cycle can be entered by sending an address with the most significant bit set to 0.

TWI-bus DATA WRITE Cycle

After the slave address with the read/write flag set to 0, a data address is written with the address/command flag (most significant bit) set to 0. The address is latched. An unlimited number of data bytes can be written in one operation with the address being automatically incremented after each write. The address will wrap to 0x00 after address 0x7F is written. Data writing will continue until a TWI stop or start condition is set by the master.

TWI-bus DATA READ Cycle

After the slave address with the read/write flag set to 0, a data address is written with the address/command flag (most significant bit) set to 0. The address is latched. To read each data byte the master then sets a start condition and sends the slave address with the read/write flag set to 1 indicating data reads will follow. An unlimited number of data bytes can be read in one operation with the address being automatically incremented after each read. The address will wrap to 0x00 after address 0x7F is read. Data reading will continue until a TWI stop or start condition is set by the master. For correct operation, the master should negative acknowledge the last byte read before setting a stop or start condition.

NOTE: It may be advantageous to set a start condition, prior to reading from the openservo, by issuing a “restart”, rather than “stop” followed by “start”- this will prevent another TWI master or device from taking control of the bus.

OpenServo TWI-bus Checked Protocol

TBD

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