eX Automotive Family FPGAs

1- 14

v3.2

5 V Tolerance of 3.3 V LVTTL I/Os Using a Tristate Buffer

Input: 3.3 V LVTTL I/Os are 5-V-input tolerant only if the non-PCI mode is used (no clamp diode).

the I/O cell with an array inverter cell and an external pull-up resistor to 5 V. The recommended configuration is

illustrated in Figure 1-14. The I/O configuration must be set to LVTTL to disable the PCI clamp diode. For the

recommended resistor value in a specific application, please contact Actel Technical Support. For more details, refer to

the Design Tips section of the Actel eX, SX-A and RT54SX-S I/Os application note.

Power Dissipation

Power consumption for eX devices can be divided into

two components: static and dynamic.

Static Power Component

The power due to standby current is typically a small

component of the overall power. Typical standby current

for eX devices is listed in Table 1-10 on page 1-12. For

is:

Dynamic Power Component

Power dissipation in CMOS devices is usually dominated

by the dynamic power dissipation. This component is

frequency-dependent and a function of the logic and the

external I/O. Dynamic power dissipation results from

charging

internal

chip

capacitance.

An

additional

component of the dynamic power dissipation is the

totem pole current in the CMOS transistor pairs. The net

effect can be associated with an equivalent capacitance

that can be combined with frequency and voltage to

represent dynamic power dissipation.

a specified frequency and voltage for each circuit

component of interest. Measurements have been made

Equivalent capacitance is frequency-independent, so the

results can be used over a wide range of operating

conditions. Equivalent capacitance values are shown

below.

CEQ Values for eX Devices

The variable and fixed capacitance of other device

components must also be taken into account when

estimating the dynamic power dissipation.

Figure 1-14 • Open-Drain Configuration for eX

E

TRIBUFF

User Internal Signal

Actel Open Drain Configuration

PAD

5 V

CEQ = Equivalent capacitance

F

= switching frequency

Combinatorial modules (Ceqcm) 1.70 pF

Sequential modules (Ceqsm)

1.70 pF

Input buffers (Ceqi)

1.30 pF

Output buffers (Ceqo)

7.40 pF

Routed array clocks (Ceqcr)

1.05 pF