255
2467S–AVR–07/09
ATmega128
SAMPLE_PRELOAD;
$2
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 76543210
JTD
– –JTRFWDRF BORF EXTRF PORF MCUCSR
Read/Write R/W R R R/W R/W R/W R/W R/W
Initial Value 0 0 0 See Bit Description
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