ATmega128 datasheet(255/386 Pages) ATMEL

255 / 386 page

255

2467S–AVR–07/09

ATmega128

SAMPLE_PRELOAD;

Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of the

input/output pins without affecting the system operation. However, the output latched are not

connected to the pins. The Boundary-scan Chain is selected as Data Register.

The active states are:

•

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

•

Shift-DR: The Boundary-scan Chain is shifted by the TCK input.

•

Update-DR: Data from the Boundary-scan Chain is applied to the output latches. However,

the output latches are not connected to the pins.

AVR_RESET; $C

The AVR specific public JTAG instruction for forcing the AVR device into the Reset mode or

releasing the JTAG Reset source. The TAP controller is not reset by this instruction. The one bit

Reset Register is selected as Data Register. Note that the Reset will be active as long as there

is a logic 'one' in the Reset Chain. The output from this chain is not latched.

The active states are:

•

Shift-DR: The Reset Register is shifted by the TCK input.

BYPASS; $F

Mandatory JTAG instruction selecting the Bypass Register for Data Register.

The active states are:

•

Capture-DR: Loads a logic “0” into the Bypass Register.

•

Shift-DR: The Bypass Register cell between TDI and TDO is shifted.

Boundary-scan

Related Register in

I/O Memory

MCU Control and

Status Register –

MCUCSR

The MCU Control and Status Register contains control bits for general MCU functions, and pro-

vides information on which reset source caused an MCU Reset.

• Bit 7 – JTD: JTAG Interface Disable

When this bit is zero, the JTAG interface is enabled if the JTAGEN fuse is programmed. If this bit

is one, the JTAG interface is disabled. In order to avoid unintentional disabling or enabling of the

JTAG interface, a timed sequence must be followed when changing this bit: The application soft-

ware must write this bit to the desired value twice within four cycles to change its value.

If the JTAG interface is left unconnected to other JTAG circuitry, the JTD bit should be set to

one. The reason for this is to avoid static current at the TDO pin in the JTAG interface.

• Bit 4 – JTRF: JTAG Reset Flag

This bit is set if a Reset is being caused by a logic one in the JTAG Reset Register selected by

the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic

zero to the flag.

Boundary-scan

Chain

The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-

tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having

off-chip connection.

Bit

765

432

JTD

–

–JTRF

WDRF

BORF

EXTRF

PORF

MCUCSR

Read/Write

R/W

Initial Value

See Bit Description

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