Datasheet

326
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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
26.3.2.1Version
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 this table.
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
below.
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 un-implemented 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 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
Part Number JTAG Part Number (Hex)
AVR USB 0x9782
Manufacturer JTAG Manufacturer ID (Hex)
Atmel 0x01F