AFS090 datasheet(160/334 Pages) MICROSEMI

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Device Architecture

2- 14 4

R e v isio n 2

For Fusion devices requiring Level 3 and/or Level 4 compliance, the board drivers connected to Fusion

I/Os need to have 10 k

Ω (or lower) output drive resistance at hot insertion, and 1 kΩ (or lower) output

drive resistance at hot removal. This is the resistance of the transmitter sending a signal to the Fusion

I/O, and no additional resistance is needed on the board. If that cannot be assured, three levels of

staging can be used to meet Level 3 and/or Level 4 compliance. Cards with two levels of staging should

have the following sequence:

1. Grounds

2. Powers, I/Os, other pins

Cold-Sparing Support

Cold-sparing means that a subsystem with no power applied (usually a circuit board) is electrically

connected to the system that is in operation. This means that all input buffers of the subsystem must

present very high input impedance with no power applied so as not to disturb the operating portion of the

system.

Pro I/O banks and standard I/O banks fully support cold-sparing.

For Pro I/O banks, standards such as PCI that require I/O clamp diodes, can also achieve cold-sparing

compliance, since clamp diodes get disconnected internally when the supplies are at 0 V.

For Advanced I/O banks, since the I/O clamp diode is always active, cold-sparing can be accomplished

either by employing a bus switch to isolate the device I/Os from the rest of the system or by driving each

advanced I/O pin to 0 V.

If Standard I/O banks are used in applications requiring cold-sparing, a discharge path from the power

supply to ground should be provided. This can be done with a discharge resistor or a switched resistor.

This is necessary because the standard I/O buffers do not have built-in I/O clamp diodes.

If a resistor is chosen, the resistor value must be calculated based on decoupling capacitance on a given

power supply on the board (this decoupling capacitor is in parallel with the resistor). The RC time

constant should ensure full discharge of supplies before cold-sparing functionality is required. The

resistor is necessary to ensure that the power pins are discharged to ground every time there is an

interruption of power to the device.

I/O cold-sparing may add additional current if the pin is configured with either a pull-up or pull down

resistor and driven in the opposite direction. A small static current is induced on each IO pin when the pin

is driven to a voltage opposite to the weak pull resistor. The current is equal to the voltage drop across

the input pin divided by the pull resistor. Please refer to Table 2-95 on page 2-171, Table 2-96 on

page 2-171, and Table 2-97 on page 2-173 for the specific pull resistor value for the corresponding I/O

standard.

For example, assuming an LVTTL 3.3 V input pin is configured with a weak Pull-up resistor, a current will

flow through the pull-up resistor if the input pin is driven low. For an LVTTL 3.3 V, pull-up resistor is ~45

Ω and the resulting current is equal to 3.3 V / 45 kΩ = 73 µA for the I/O pin. This is true also when a

weak pull-down is chosen and the input pin is driven high. Avoiding this current can be done by driving

the input low when a weak pull-down resistor is used, and driving it high when a weak pull-up resistor is

used.

In Active and Static modes, this current draw can occur in the following cases:

•

Input buffers with pull-up, driven low

•

Input buffers with pull-down, driven high

•

Bidirectional buffers with pull-up, driven low

•

Bidirectional buffers with pull-down, driven high

•

Output buffers with pull-up, driven low

•

Output buffers with pull-down, driven high

•

Tristate buffers with pull-up, driven low

•

Tristate buffers with pull-down, driven high

AFS090 数据表(PDF) 160 Page - Microsemi Corporation

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