Rainbow Electronics Atmega169L - Manual (Page 233)

233
ATmega169V/L
2514A–AVR–08/02
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 unimple-
mented 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 settings for the clock options, the part will remain reset for a reset time-out
period (refer to “Clock Sources” on page 24) after releasing the Reset Register. The out-
put from this data register is not latched, so the reset will take place immediately, as
shown in Figure 107.
Figure 107. Reset Register
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 connections.
See “Boundary-scan Chain” on page 235 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 data sheet, 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.
D Q
From
TDI
ClockDR · AVR_RESET
To
TDO
From Other Internal and
External Reset Sources
Internal reset