Rainbow Electronics ATmega64L - Manual (Page 223)

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ATmega16(L)
2466B–09/01
SAMPLE_PRELOAD; $2 Mandatory JTAG instruction for pre-loading the output latches and talking a snap-shot of
the input/output pins without affecting the system operation. However, the output latches
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 instruc-
tion. 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 provides information on which reset source caused an MCU reset.
• Bits 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 software must write this bit to the desired value twice within four
cycles to change its value.
• 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.
Bit 7 6 5 4 3 2 1 0
JTD ISC2 – JTRF WDRF BORF EXTRF PORF MCUCSR
Read/Write R/W R/W R R/W R/W R/W R/W R/W
Initial Value 0 0 0 See Bit Description