What is a TAP controller?

A TAP controller is a 16-state machine, programmed by the Test Mode Select (TMS) and Test Clock (TCK) inputs, which controls the flow of data bits to the Instruction Register (IR) and the Data Registers (DR). The TAP Controller can be thought of as the control center of a boundary-scan device.

What is TAP controller and port?

The test access ports (TAP), which define the bus protocol of boundary scan, are the additional I/O pins needed for each chip employing Std. 1149.1a. The TAP controller is a 16-state final state machine that controls each step of the operations of boundary scan.

What is a JTAG controller?

​JTAG is a common hardware interface that provides your computer with a way to communicate directly with the chips on a board. It was originally developed by a consortium, the Joint (European) Test Access Group, in the mid-80s to address the increasing difficulty of testing printed circuit boards (PCBs).

What is JTAG chain?

One or more devices through which programming and/or configuration data are passed from device to device via the Joint Test Action Group (JTAG) Boundary-Scan Test (BST) circuitry. A JTAG chain can consist of only one device.

Which input controls the sequence of states through which the JTAG TAP state machine passes?

The TAP controller is a finite state machine that responds to changes at the TMS and TCK signals of the TAP and controls the sequence of operations of the circuitry defined by standard. It also controls the scanning of data into the various registers of the JTAG architecture.

What are the mandatory instructions in JTAG?

The only JTAG instructions which are defined and required by the IEEE 1149.1 standard are BYPASS, EXTEST and SAMPLE/PRELOAD. BYPASS is defined as all bits in the IR as all ones. EXTEST is defined as all bits in the IR as all zeros.

What is the purpose of JTAG?

JTAG allows device programmer hardware to transfer data into internal non-volatile device memory (e.g. CPLDs). Some device programmers serve a double purpose for programming as well as debugging the device.

What is TAP controller in JTAG?

TAP Controller It controls the JTAG operation. It is basically a 16-state Finite State Machine (FSM) whose state transitions are controlled by the TMS signal as shown in Figure 2. The TAP controller can change state only at the rising edge of TCK and the next state is determined by the logic level of TMS and the present state.

What are the analogous paths through the JTAG state machine?

Two analogous paths through the state machine are used to capture and/or update data by scanning through the instruction register (IR) or through a data register (DR). The JTAG state machine is depicted in Figure 2 below.

What signals are used in JTAG interface?

The JTAG interface, collectively known as a Test Access Port, or TAP, uses the following signals to support the operation of boundary scan. TCK (Test Clock) – this signal synchronizes the internal state machine operations.

How are transtransitions determined in JTAG?

Transitions are determined by the state of TMS on the rising edge of TCK. Two analogous paths through the state machine are used to capture and/or update data by scanning through the instruction register (IR) or through a data register (DR). The JTAG state machine is depicted in Figure 2 below.