Bypass Register
The Bypass Register consists of a single Shift Register stage. When the Bypass Regis-
ter is selected as path between TDI and TDO, the register is reset to 0 when leaving the
Capture-DR controller state. The Bypass Register can be used to shorten the scan
chain on a system when the other devices are to be tested.
Device Identification Register Figure 131 shows the structure of the Device Identification Register.
Figure 131. The Format of the Device Identification Register
LSB
MSB
Bit
31
28
27
12
11
1
0
Device ID
Version
Part Number
Manufacturer ID
1
4 bits
16 bits
11 bits
1-bit
Version
Version is a 4-bit number identifying the revision of the component. The JTAG version
number follows the revision of the device. Revision A is 0x0, revision B is 0x1 and so on.
Part Number
The part number is a 16-bit code identifying the component. The JTAG Part Number for
ATmega640/1280/1281/2560/2561 is listed in Table 129.
Table 129. AVR JTAG Part Number
Part Number
ATmega640
ATmega1280
ATmega1281
ATmega2560
ATmega2561
JTAG Part Number (Hex)
0x9607
0x9703
0x9704
0x9801
0x9802
Manufacturer ID
The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufac-
turer ID for ATMEL is listed in Table 130.
Table 130. Manufacturer ID
Manufacturer
JTAG Manufactor ID (Hex)
ATMEL
0x01F
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 settings for the clock options, the part will remain reset for a reset time-out
period (see “Clock Sources” on page 40) 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 132.
302
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
