ATMEGA640 datasheet(297/436 Pages) MICROCHIP

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297

ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Figure 28-2. Reset Register

28.3.4

Boundary-scan Chain

The Boundary-scan Chain has the capability of driving and observing the logic levels on the digital I/O pins, as well

as the boundary between digital and analog logic for analog circuitry having off-chip connections.

See “Boundary-scan Chain” on page 298 for a complete description.

28.4 Boundary-scan Specific JTAG Instructions

The Instruction Register is 4-bit wide, supporting up to 16 instructions. Listed below are the JTAG instructions use-

ful for Boundary-scan operation. Note that the optional HIGHZ instruction is not implemented, but all outputs with

tri-state capability can be set in high-impedant state by using the AVR_RESET instruction, since the initial state for

all port pins is tri-state.

As a definition in this datasheet, the LSB is shifted in and out first for all Shift Registers.

The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes which

Data Register is selected as path between TDI and TDO for each instruction.

28.4.1

EXTEST; 0x0

Mandatory JTAG instruction for selecting the Boundary-scan Chain as Data Register for testing circuitry external to

the AVR package. For port-pins, Pull-up Disable, Output Control, Output Data, and Input Data are all accessible in

the scan chain. For Analog circuits having off-chip connections, the interface between the analog and the digital

logic is in the scan chain. The contents of the latched outputs of the Boundary-scan chain is driven out as soon as

the JTAG IR-Register is loaded with the EXTEST instruction.

The active states are:

•

Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain

•

Shift-DR: The Internal Scan Chain is shifted by the TCK input

•

Update-DR: Data from the scan chain is applied to output pins

28.4.2

IDCODE; 0x1

Optional JTAG instruction selecting the 32-bit ID-Register as Data Register. The ID-Register consists of a version

number, a device number and the manufacturer code chosen by JEDEC. This is the default instruction after power-

up.

The active states are:

•

Capture-DR: Data in the IDCODE Register is sampled into the Boundary-scan Chain

•

Shift-DR: The IDCODE scan chain is shifted by the TCK input

From

TDI

ClockDR · AVR_RESET

TDO

From Other Internal and

External Reset Sources

Internal reset

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