ATmega16U4 datasheet(317/408 Pages) ATMEL

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7766D–AVR–01/09

ATmega16U4/ATmega32U4

Figure 25-2. TAP Controller State Diagram

25.3

TAP Controller

The TAP controller is a 16-state finite state machine that controls the operation of the Boundary-

scan circuitry, JTAG programming circuitry, or On-chip Debug system. The state transitions

depicted in Figure 25-2 depend on the signal present on TMS (shown adjacent to each state

transition) at the time of the rising edge at TCK. The initial state after a Power-on Reset is Test-

Logic-Reset.

As a definition in this document, the LSB is shifted in and out first for all Shift Registers.

Assuming Run-Test/Idle is the present state, a typical scenario for using the JTAG interface is:

• At the TMS input, apply the sequence 1, 1, 0, 0 at the rising edges of TCK to enter the Shift

Instruction Register – Shift-IR state. While in this state, shift the four bits of the JTAG

instructions into the JTAG Instruction Register from the TDI input at the rising edge of TCK.

The TMS input must be held low during input of the 3 LSBs in order to remain in the Shift-IR

state. The MSB of the instruction is shifted in when this state is left by setting TMS high.

While the instruction is shifted in from the TDI pin, the captured IR-state 0x01 is shifted out on

the TDO pin. The JTAG Instruction selects a particular Data Register as path between TDI

and TDO and controls the circuitry surrounding the selected Data Register.

Test-Logic-Reset

Run-Test/Idle

Shift-DR

Exit1-DR

Pause-DR

Exit2-DR

Update-DR

Select-IR Scan

Capture-IR

Shift-IR

Exit1-IR

Pause-IR

Exit2-IR

Update-IR

Select-DR Scan

Capture-DR

ATmega16U4 Datasheet(PDF) 317 Page - ATMEL Corporation