ATmega128 datasheet(253/386 Pages) ATMEL

253 / 386 page

253

2467S–AVR–07/09

ATmega128

Bypass Register

The Bypass Register consists of a single Shift Register stage. When the Bypass Register 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

Figure 122 shows the structure of the Device Identification Register.

Figure 122. The Format of the Device Identification Register

Version

Version is a 4-bit number identifying the revision of the component. The JTAG version number

follows the revision of the device, and wraps around at revision P (0xF). Revision A and Q is

0x0, revision B and R is 0x1 and so on.

Part Number

The part number is a 16-bit code identifying the component. The JTAG Part Number for

ATmega128 is listed in Table 100.

Manufacturer ID

The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID

for ATMEL is listed in Table 101.

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 37) 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 123.

MSB

LSB

Bit

Device ID

Version

Part Number

Manufacturer ID

4 bits

16 bits

11 bits

1-bit

Table 100. AVR JTAG Part Number

Part Number

JTAG Part Number (Hex)

ATmega128

0x9702

Table 101. Manufacturer ID

Manufacturer

JTAG Manufacturer ID (Hex)

ATMEL

0x01F

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