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ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
25.4.3
SAMPLE_PRELOAD; 0x2
Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of the input/output pins with-
out 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
25.4.4
AVR_RESET; 0xC
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
25.4.5
BYPASS; 0xF
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
25.5
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 connection.
25.5.1
Scanning the Digital Port Pins
consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn – function, and a bi-directional pin cell
that combines the three signals Output Control – OCxn, Output Data – ODxn, and Input Data – IDxn, into only a
two-stage Shift Register. The port and pin indexes are not used in the following description
When no alternate port function is present, the Input Data – ID – corresponds to the PINxn Register value (but ID
has no synchronizer), Output Data corresponds to the PORT Register, Output Control corresponds to the Data
Direction – DD Register, and the Pull-up Enable – PUExn – corresponds to logic expression PUD DDxn
PORTxn.
chain read the actual pin value. For Analog function, there is a direct connection from the external pin to the analog
circuit, and a scan chain is inserted on the interface between the digital logic and the analog circuitry.