Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 262
24. JTAG interface and on-chip debug system
24.1 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 Atmel Studio
24.2 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 ”Programming via the JTAG
Interface” on page 314 and ”IEEE 1149.1 (JTAG) Boundary-scan” on page 268, respectively. The On-chip
Debug support is considered being private JTAG instructions, and distributed within Microchip and to selected
third party vendors only.
Figure 24-1 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, 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.
24.3 TAP – Test Access Port
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
 TCK: Test Clock. JTAG operation is synchronous to TCK