Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P

 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 269

25.3 Data Registers

The Data Registers relevant for Boundary-scan operations are:

 Bypass Register

 Device Identification Register

 Reset Register

 Boundary-scan Chain

25.3.1 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

25.3.2 Device Identification Register

Figure 25-1 shows the structure of the Device Identification Register.

Figure 25-1. 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. 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

ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P is listed in Table 27-6 on page 298.

Manufacturer ID

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

27-6 on page 298.

25.3.3 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 (refer to ”Clock Sources” on page 39) after releasing the Reset

Figure 25-2 on page 270.

MSB

LSB

Bit

31 28 27 12 11 1 0

Device ID Version Part Number Manufacturer ID 1

4 bits 16 bits 11 bits 1-bit