Datasheet

301
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
JTAG Interface and
On-chip Debug
System
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
®
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 Program-
ming via the JTAG interface, and using the Boundary-scan Chain can be found in the
sections “Programming via the JTAG Interface” on page 361 and “IEEE 1149.1 (JTAG)
Boundary-scan” on page 308, respectively. The On-chip Debug support is considered
being private JTAG instructions, and distributed within ATMEL and to selected third
party vendors only.
Figure 128 shows a block diagram of the JTAG interface and the On-chip Debug sys-
tem. 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 Reg-
isters 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, Bypass Register, and the Boundary-scan Chain are the Data Registers
used for board-level testing. The JTAG Programming Interface (actually consisting 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.