Intel 21555 user manual Test Access Port Controller, Initialization

JTAG Test Port

13.2Test Access Port Controller

The test access port controller is a finite-state machine that interprets IEEE 1149.1 protocols received through the tms signal. The state transitions in the controller are caused by the tms signal on the rising edge of tck. In each state, the controller generates appropriate clock and control signals that control the operation of the test features. After entry into a state, test feature operations are initiated on the rising edge of tck.

13.2.1Initialization

The test access port controller and the instruction register output latches are initialized and JTAG is disabled while the trst_l input is asserted low (see Figure 26). While signal trst_l is low, the test access port controller enters the test-logic reset state. This results in the instruction register being reset which holds the bypass register instruction. During test-logic reset state, all JTAG test logic is disabled, and the device performs normal functions. The test access port controller leaves this state only after trst_l (low) goes high and an appropriate JTAG test operation sequence is sent on the tms and tck pins.

For the 21555 to operate properly, the JTAG logic must be reset. The controller will reset:

•Asynchronously with the assertion of trst_l .

•Synchronously after five tck clock cycles, with tms held high.

Figure 26. Signal trst_l States

trst_l

JTAG Reset

JTAG Enabled

A7805-01

Note: Prior to normal 21555 operation, this signal must be strobed low or pulled low with a 1kΩ resistor.

112 21555 Non-Transparent PCI-to-PCI Bridge User Manual