Datasheet

321
7766F–AVR–11/10
ATmega16/32U4
26.3.1 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 Register can be used to shorten the scan chain on a system when
the other devices are to be tested.
26.3.2 Device Identification Register
Figure 26-1 shows the structure of the Device Identification Register.
Figure 26-1. The Format of the Device Identification Register
26.3.2.1 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.
26.3.2.2 Part Number
The part number is a 16-bit code identifying the component. The JTAG Part Number for
ATmega16U4/ATmega32U4 is listed in Table 26-1.
26.3.2.3 Manufacturer ID
The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID
for ATMEL is listed in Table 26-2.
26.3.3 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 set-
tings for the clock options, the part will remain reset for a reset time-out period (refer to “Clock
Sources” on page 28) 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 26-2.
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
Table 26-1. AVR JTAG Part Number
Part Number JTAG Part Number (Hex)
AVR USB 0x9782
Table 26-2. Manufacturer ID
Manufacturer JTAG Manufacturer ID (Hex)
ATM EL 0x 01 F