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GR716-DS-UM, May 2019, Version 1.29
91
www.cobham.com/gaisler
GR716
9
Crystal (XO) Oscillator
9.1
Overview
The on-chip crystal oscillator (XO) contains all
active
oscillator parts, and a crystal (XTAL) is added
on PCB. It provides a clock output signal as a 3.3V CMOS square-wave output.
9.2
Operation
9.2.1
System overview
The on-chip XO supports generation of an accurate XTAL-based oscillator clock signal, where its
output signal can be directly connected to the system clock input on the LEON3FT microcontroller.
This clock signal can also be arbitrarily used on PCB. If a precision XO on PCB is needed, the on-
chip XO can then be made to draw negligible power if desired.
9.2.2
Detailed description
The XO block is supplied by the Microcontroller core voltage, VDD_CORE (1.8V). The oscillator
output is a 3.3V CMOS output and is available on an external pin. The XO block requires an external
crystal on PCB (parallel-resonant fundamental-tone AC-cut XTAL). The XTAL two terminals are to
be connected directly to the two external XO pins, and a capacitor to ground on each XO pin is added
for fine-tuning of the oscillator frequency. The range of supported XTAL frequencies is 5 to 25 MHz,
where 5MHz is recommended for low-power applications and up to 25MHz for high-performance
applications. See chapter TBD for the details how to implement the XO interface in PCB design.
In applications where an external high-precision oscillator on PCB needs to be used (TCXO, OCXO,
etc), a 3.3V CMOS-compatible oscillator signal should be fed into the system clock input. All three
external pins on the on-chip XO can then be left open. To minimize the XO current consumption, an
detector is build-in to disable the XO if no external XTAL is connected to the external XO pins.
9.2.3
Typical crystal configurations
This section specifies a number of typical crystal configurations for the GR716 device.
C
X0
C
X1
XO_X0
XO_X1
XTAL
GND
GND