ATmega16 datasheet(224/315 Pages) ATMEL

224 / 315 page

224

ATmega16(L)

2466E–AVR–10/02

Figure 115. Reset Register

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 226 for a complete description.

Boundary-scan Specific

JTAG Instructions

The instruction register is 4-bit wide, supporting up to 16 instructions. Listed below are

the JTAG instructions useful 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 data sheet, 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.

EXTEST; $0

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 cir-

cuits 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.

IDCODE; $1

Optional JTAG instruction selecting the 32-bit ID register as Data Register. The ID regis-

ter 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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