FG256 datasheet(13/108 Pages) ACTEL

13 / 108 page

SX-A Family FPGAs

v5.3

1-9

Boundary-Scan Testing (BST)

All SX-A devices are IEEE 1149.1 compliant and offer

superior diagnostic and testing capabilities by providing

Boundary Scan Testing (BST) and probing capabilities.

The BST function is controlled through the special JTAG

pins (TMS, TDI, TCK, TDO, and TRST). The functionality of

the JTAG pins is defined by two available modes:

Dedicated and Flexible. TMS cannot be employed as a

user I/O in either mode.

Dedicated Mode

In Dedicated mode, all JTAG pins are reserved for BST;

designers cannot use them as regular I/Os. An internal

pull-up resistor is automatically enabled on both TMS

and TDI pins, and the TMS pin will function as defined in

the IEEE 1149.1 (JTAG) specification.

To select Dedicated mode, the user must reserve the

JTAG pins in Actel’s Designer software. Reserve the JTAG

pins by checking the Reserve JTAG box in the Device

Selection Wizard (Figure 1-12).

The default for the software is Flexible mode; all boxes

are unchecked. Table 1-5 lists the definitions of the

options in the Device Selection Wizard.

Flexible Mode

In Flexible mode, TDI, TCK, and TDO may be employed as

either user I/Os or as JTAG input pins. The internal

resistors on the TMS and TDI pins are not present in

flexible JTAG mode.

To select the Flexible mode, uncheck the Reserve JTAG

box in the Device Selection Wizard dialog in the Actel

Designer software. In Flexible mode, TDI, TCK, and TDO

pins may function as user I/Os or BST pins. The

functionality is controlled by the BST Test Access Port

(TAP) controller. The TAP controller receives two control

inputs, TMS and TCK. Upon power-up, the TAP controller

enters the Test-Logic-Reset state. In this state, TDI, TCK,

and TDO function as user I/Os. The TDI, TCK, and TDO are

transformed from user I/Os into BST pins when a rising

edge on TCK is detected while TMS is at logic low. To

return to Test-Logic Reset state, TMS must be high for at

least five TCK cycles. An external 10 k pull-up resistor

to VCCI should be placed on the TMS pin to pull it

High by default.

Table 1-6

describes

the

different

configuration

requirements of BST pins and their functionality in

different modes.

TRST Pin

The TRST pin functions as a dedicated Boundary-Scan

Reset pin when the Reserve JTAG Test Reset option is

selected as shown in Figure 1-12. An internal pull-up

resistor is permanently enabled on the TRST pin in this

mode. Actel recommends connecting this pin to ground

in normal operation to keep the JTAG state controller in

the Test-Logic-Reset state. When JTAG is being used, it

can be left floating or can be driven high.

When the Reserve JTAG Test Reset option is not

selected, this pin will function as a regular I/O. If unused

as an I/O in the design, it will be configured as a tristated

output.

Figure 1-12 • Device Selection Wizard

Table 1-5 • Reserve Pin Definitions

Function

Reserve JTAG

Keeps pins from being used and

changes the behavior of JTAG pins (no

pull-up on TMS)

Reserve

JTAG

Test

Reset

Regular I/O or JTAG reset with an

internal pull-up

Reserve Probe

Keeps pins from being used or regular

I/O

Table 1-6 • Boundary-Scan Pin Configurations and

Functions

Mode

Designer

"Reserve JTAG"

Selection

TAP Controller

State

Dedicated (JTAG)

Checked

Any

Flexible (User I/O)

Unchecked

Test-Logic-Reset

Flexible (JTAG)

Unchecked

Any EXCEPT Test-

Logic-Reset

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