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22. JTAG Interface and On-chip Debug System
22.1 Features
• JTAG (IEEE std. 1149.1 Compliant) Interface
• Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard
• Debugger Access to:
– All Internal Peripheral Units
– Internal and External RAM
– The Internal Register File
–Program Counter
– EEPROM and Flash Memories
• Extensive On-chip Debug Support for Break Conditions, Including
– AVR Break Instruction
– Break on Change of Program Memory Flow
– Single Step Break
– Program Memory Break Points on Single Address or Address Range
– Data Memory Break Points on Single Address or Address Range
• Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
• On-chip Debugging Supported by AVR Studio
®
22.2 Overview
The AVR IEEE std. 1149.1 compliant JTAG interface can be used for:
• Testing PCBs by using the JTAG Boundary-scan capability
• Programming the non-volatile memories, Fuses and Lock bits
• On-chip debugging
A brief description is given in the following sections. Detailed descriptions for Programming via
the JTAG interface, and using the Boundary-scan Chain can be found in the sections “JTAG
Programming Overview” on page 352 and “Boundary-scan IEEE 1149.1 (JTAG)” on page 300,
respectively. The On-chip Debug support is considered being private JTAG instructions, and dis-
tributed within ATMEL and to selected third party vendors only.
Figure 22-1 shows a block diagram of the JTAG interface and the On-chip Debug system. The
TAP Controller is a state machine controlled by the TCK and TMS signals. The TAP Controller
selects either the JTAG Instruction Register or one of several Data Registers as the scan chain
(Shift Register) between the TDI – input and TDO – output. The Instruction Register holds JTAG
instructions controlling the behavior of a Data Register.
The ID-Register (IDentifier Register), Bypass Register, and the Boundary-scan Chain are the
Data Registers used for board-level testing. The JTAG Programming Interface (actually consist-
ing of several physical and virtual Data Registers) is used for serial programming via the JTAG
interface. The Internal Scan Chain and Break Point Scan Chain are used for On-chip debugging
only.
22.3 Test Access Port – TAP
The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology, these pins
constitute the Test Access Port – TAP. These pins are:
• TMS: Test mode select. This pin is used for navigating through the TAP-controller state
machine.
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