ProASIC3 Flash Family FPGAs
Revision 13 3-3
should be treated as a sensitive asynchronous signal. When defining pin placement and board layout,
simultaneously switching outputs (SSOs) and their effects on sensitive asynchronous pins must be
considered.
Unused FF or I/O pins are tristated with weak pull-up. This default configuration applies to both
Flash*Freeze mode and normal operation mode. No user intervention is required.
JTAG Pins
Low power flash devices have a separate bank for the dedicated JTAG pins. The JTAG pins can be run
at any voltage from 1.5 V to 3.3 V (nominal). VCC must also be powered for the JTAG state machine to
operate, even if the device is in bypass mode; VJTAG alone is insufficient. Both VJTAG and VCC to the
part must be supplied to allow JTAG signals to transition the device. Isolating the JTAG power supply in a
separate I/O bank gives greater flexibility in supply selection and simplifies power supply and PCB
design. If the JTAG interface is neither used nor planned for use, the VJTAG pin together with the TRST
pin could be tied to GND.
TCK Test Clock
Test clock input for JTAG boundary scan, ISP, and UJTAG. The TCK pin does not have an internal pull-
up/-down resistor. If JTAG is not used, Microsemi recommends tying off TCK to GND through a resistor
placed close to the FPGA pin. This prevents JTAG operation in case TMS enters an undesired state.
Note that to operate at all VJTAG voltages, 500 to 1 k will satisfy the requirements. Refer to Tab l e 1
for more information.
TDI Test Data Input
Serial input for JTAG boundary scan, ISP, and UJTAG usage. There is an internal weak pull-up resistor
on the TDI pin.
TDO Test Data Output
Serial output for JTAG boundary scan, ISP, and UJTAG usage.
TMS Test Mode Select
The TMS pin controls the use of the IEEE 1532 boundary scan pins (TCK, TDI, TDO, TRST). There is an
internal weak pull-up resistor on the TMS pin.
TRST Boundary Scan Reset Pin
The TRST pin functions as an active low input to asynchronously initialize (or reset) the boundary scan
circuitry. There is an internal weak pull-up resistor on the TRST pin. If JTAG is not used, an external pull-
down resistor could be included to ensure the test access port (TAP) is held in reset mode. The resistor
values must be chosen from Table 1 and must satisfy the parallel resistance value requirement. The
values in Tab l e 1 correspond to the resistor recommended when a single device is used, and the
equivalent parallel resistor when multiple devices are connected via a JTAG chain.
In critical applications, an upset in the JTAG circuit could allow entrance to an undesired JTAG state. In
such cases, Microsemi recommends tying off TRST to GND through a resistor placed close to the FPGA
pin.
Note that to operate at all VJTAG voltages, 500 to 1 k will satisfy the requirements.
Table 1 • Recommended Tie-Off Values for the TCK and TRST Pins
VJTAG Tie-Off Resistance
VJTAG at 3.3 V 200 to 1 k
VJTAG at 2.5 V 200 to 1 k
VJTAG at 1.8 V 500 to 1 k
VJTAG at 1.5 V 500 to 1 k
Notes:
1. Equivalent parallel resistance if more than one device is on the JTAG chain
2. The TCK pin can be pulled up/down.
3. The TRST pin is pulled down.
