STK500 Microchip

AVR068: STK500 Communication Protocol Features 8-bit Interfaces both STK500 and AVRISP Microcontrollers Supports STK500 FW 2.XX 1 Introduction Application Note This document describes the 2.0 version of the communication protocol between the Atmel STK500 and the PC controlling the STK500. The firmware is distributed with AVR Studio 4.11 build 401 or later. The definition of all commands, responses, parameters and other defined values can be found in the file command.h. This file can be downloaded from the Atmel web site. Most device specific values can be found in the xml files for each part. The xml files are distributed with AvrStudio 4.11 or later. Download the latest AVR Studio 4.x from the Atmel web site, 2 Communication interface The communication between the STK500 and the PC is done over RS232 (PC COM port). The STK500 uses: 115.2kbps, 8 data bits, 1stop bits, no parity. The PC should be set up similarly for the communication to work. 3 Message Format All commands and responses share a common message format: Figure 3-1. Message format. MESSAGE START SEQUENCE NUMBER MESSAGE SIZE TOKEN MESSAGE BODY CHECKSUM Table 3-1. Common message format fields Parameter Name Size/Format Description MESSAGE START 1 byte Always 0x1B SEQUENCE NUMBER 1 byte Incremented by one for each message sent. Wraps to zero after 0xFF is reached. (1) MESSAGE SIZE 2 bytes, MSB first Size of the message body TOKEN 1 byte Always 0x0E MESSAGE BODY MESSAGE SIZE bytes Message body, from 0 to 65535 bytes (1) CHECKSUM 1 byte Uses all characters in message including MESSAGE START and MESSAGE BODY. XOR of all bytes. Note: 1. The current STK500 firmware can only handle messages with a message body of maximum of 275 bytes. The host (PC) sends commands, the STK500 responds with answers. One command will result in one answer. STK500 cannot send a message to the host that is not an answer to a received command. The sequence number for an answer is always the same as for the corresponding command. 4 Protocol Layer State Table This section describes the state machine in the PC software that handles incoming packets from the STK500. When an incoming packet is expected, the state machine is initialized to the Start state. 2 AVR068 2591C-AVR-06/06