AFS090 datasheet(243/334 Pages) MICROSEMI

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Fusion Family of Mixed Signal FPGAs

R e visio n 2

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VAREF

Analog Reference Voltage

The Fusion device can be configured to generate a 2.56 V internal reference voltage that can be used by

the ADC. While using the internal reference, the reference voltage is output on the VAREF pin for use as

a system reference. If a different reference voltage is required, it can be supplied by an external source

and applied to this pin. The valid range of values that can be supplied to the ADC is 1.0 V to 3.3 V. When

VAREF is internally generated by the Fusion device, a bypass capacitor must be connected from this pin

to ground. The value of the bypass capacitor should be between 3.3 µF and 22 µF, which is based on the

needs of the individual designs. The choice of the capacitor value has an impact on the settling time it

takes the VAREF signal to reach the required specification of 2.56 V to initiate valid conversions by the

ADC. If the lower capacitor value is chosen, the settling time required for VAREF to achieve 2.56 V will

be shorter than when selecting the larger capacitor value. The above range of capacitor values supports

the accuracy specification of the ADC, which is detailed in the datasheet. Designers choosing the smaller

capacitor value will not obtain as much margin in the accuracy as that achieved with a larger capacitor

value. Depending on the capacitor value selected in the Analog System Builder, a tool in Libero IDE, an

automatic delay circuit will be generated using logic tiles available within the FPGA to ensure that VAREF

has achieved the 2.56 V value. Microsemi recommends customers use 10 µF as the value of the bypass

capacitor. Designers choosing to use an external VAREF need to ensure that a stable and clean VAREF

source is supplied to the VAREF pin before initiating conversions by the ADC. Designers should also

make sure that the ADCRESET signal is deasserted before initiating valid conversions.2

User Pins

I/O

User Input/Output

The I/O pin functions as an input, output, tristate, or bidirectional buffer. Input and output signal levels are

compatible with the I/O standard selected. Unused I/O pins are configured as inputs with pull-up

resistors.

During programming, I/Os become tristated and weakly pulled up to VCCI. With the VCCI and VCC

supplies continuously powered up, when the device transitions from programming to operating mode, the

I/Os get instantly configured to the desired user configuration.

Unused I/Os are configured as follows:

•

Output buffer is disabled (with tristate value of high impedance)

•

Input buffer is disabled (with tristate value of high impedance)

•

Weak pull-up is programmed

Axy

Analog Input/Output

Analog I/O pin, where x is the analog pad type (C = current pad, G = Gate driver pad, T = Temperature

pad, V = Voltage pad) and y is the Analog Quad number (0 to 9). There is a minimum 1 M

Ω to ground on

AV, AC, and AT. This pin can be left floating when it is unused.

ATRTNx

Temperature Monitor Return

AT returns

are the returns for the temperature sensors. The cathode terminal of the external diodes

should be connected to these pins. There is one analog return pin for every two Analog Quads. The x in

the ATRTNx designator indicates the quad pairing (x = 0 for AQ1 and AQ2, x = 1 for AQ2 and AQ3, ...,

= 4 for AQ8 and AQ9). The signals that drive these pins are called out as ATRETURNxy in the software

(where x and y refer to the quads that share the return signal). ATRTN is internally connected to ground.

It can be left floating when it is unused. The maximum capacitance allowed across the AT pins is 500 pF.

Globals

GL I/Os have access to certain clock conditioning circuitry (and the PLL) and/or have direct access to the

global network (spines). Additionally, the global I/Os can be used as Pro I/Os since they have identical

capabilities. Unused GL pins are configured as inputs with pull-up resistors. See more detailed

descriptions of global I/O connectivity in the "Clock Conditioning Circuits" section on page 2-23.

2. The ADC is functional with an external reference down to 1V, however to meet the performance parameters highlighted in the

datasheet refer to the VAREF specification in Table 3-2 on page 3-3.

AFS090 Datasheet(PDF) 243 Page - Microsemi Corporation

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