AFS090 datasheet(37/334 Pages) MICROSEMI

37 / 334 page

Fusion Family of Mixed Signal FPGAs

R e visio n 2

2 - 21

Crystal Oscillator

The Crystal Oscillator (XTLOSC) is source that generates the clock from an external crystal. The output

of XTLOSC CLKOUT signal can be selected as an input to the PLL. Refer to the "Clock Conditioning

Circuits" section for more details. The XTLOSC can operate in normal operations and Standby mode

(RTC is running and 1.5 V is not present).

In normal operation, the internal FPGA_EN signal is '1' as long as 1.5 V is present for VCC. As such,

the internal enable signal, XTL_EN, for Crystal Oscillator is enabled since FPGA_EN is asserted. The

XTL_MODE has the option of using MODE or RTC_MODE, depending on SELMODE.

During Standby, 1.5 V is not available, as such, and FPGA_EN is '0'. SELMODE must be asserted in

order for XTL_EN to be enabled; hence XTL_MODE relies on RTC_MODE. SELMODE and RTC_MODE

must be connected to RTCXTLSEL and RTCXTLMODE from the AB respectively for correct operation

during Standby (refer to the "Real-Time Counter System" section on page 2-33 for a detailed

description).

The Crystal Oscillator can be configured in one of four modes:

•

RC network, 32 KHz to 4 MHz

•

Low gain, 32 to 200 KHz

•

Medium gain, 0.20 to 2.0 MHz

•

High gain, 2.0 to 20.0 MHz

In RC network mode, the XTAL1 pin is connected to an RC circuit, as shown in Figure 2-16 on

page 2-19. The XTAL2 pin should be left floating. The RC value can be chosen based on Figure 2-18 for

any desired frequency between 32 KHz and 4 MHz. The RC network mode can also accommodate an

external clock source on XTAL1 instead of an RC circuit.

In Low gain, Medium gain, and High gain, an external crystal component or ceramic resonator can be

added onto XTAL1 and XTAL2, as shown in Figure 2-16 on page 2-19. In the case where the Crystal

Oscillator block is not used, the XTAL1 pin should be connected to GND and the XTAL2 pin should be left

floating.

Note:

*Internal signal—does not exist in macro.

Figure 2-17 • XTLOSC Macro

XT LOSC

CLKOUT

XT L

MODE[1:0]

RTC_MODE[1:0]

SELMODE

FPGA_EN*

XTL_EN*

XTL_MODE*

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