Datasheet

256
2490R–AVR–02/2013
ATmega64(L)
Reset Register The Reset Register is a Test Data Register used to reset the part. Since the AVR tri-states port
pins when reset, the Reset Register can also replace the function of the unimplemented optional
JTAG instruction HIGHZ.
A high value in the Reset Register corresponds to pulling the External Reset low. The part is
reset as long as there is a high value present in the Reset Register. Depending on the Fuse set-
tings for the clock options, the part will remain reset for a Reset Time-out Period (refer to “Clock
Sources” on page 38) after releasing the Reset Register. The output from this data register is not
latched, so the reset will take place immediately, as shown in Figure 126.
Figure 126. Reset Register
Boundary-scan Chain The Boundary-scan Chain has the capability of driving and observing the logic levels on the dig-
ital 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 258 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 datasheet, 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; 0x0 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 circuits having Off-chip
connections, the interface between the analog and the digital logic is in the scan chain. The con-
tents 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.
DQ
From
TDI
ClockDR · AVR_RESET
To
TDO
From Other Internal and
External Reset Sources
Internal Reset