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GR716-DS-UM, May 2019, Version 1.29
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www.cobham.com/gaisler
GR716
2.3.2
XO oscillator
The oscillator (XO) is supplied by the LEON3FT microcontroller core voltage, VDD_CORE (1.8V).
The oscillator output is a 3.3V CMOS output and is available on an external pin.
2.3.3
PLL
The PLL is supplied by 1.8V from an internal LDO, which should have an external decoupling capac-
itor on the PLL supply pin (1.8V). This supply pin shall be left open, with exception of this decou-
pling capacitor. The PLL provides several internal clock outputs, typically used as clock for the
SpaceWire interface, etc. The PLL reference-clock input is a 3.3V CMOS input, to which the XO-
oscillator clock output can be directly connected, or any other clock signal generated on PCB fulfill-
ing the electrical specification of this input. The PLL reference-clock input is allowed to be asynchro-
nous to any other clocks in the GR716 microcontroller.
2.3.4
Voltage reference
The reference blocks are supplied by VDDA_REF. This supply needs to have the best voltage integ-
rity on the chip. Therefore, no fast load-current steps are present in any of the on-chip blocks using
this supply. It is essential that especially this supply has good PCB decoupling/filtering (across
VDDA_REF and VSSA_REF) in order to not feed external disturbances from PCB supplies into this
supply. The analog internal references are generated in two steps. First, a reference voltage is gener-
ated by an on-chip band-gap reference, which should have an external decoupling capacitor on the
VREF pin. Alternatively, the on-chip reference block can be turned off; then, an external reference
voltage must be applied to this pin (including the right decoupling capacitance needed in that voltage-
reference implementation). Second, this reference voltage is buffered and put out on the VREFBUF
pin. This reference voltage is also used by an internal current generator, which puts this voltage across
an external reference resistor, RREF on PCB, to generate a precision reference current. Since this ref-
erence current is used to generate both the current reference to each DAC and the internal bias cur-
rents required by several other on-chip blocks, RREF can
not
be chosen arbitrarily to get any desired
DAC full-scale value; it shall be 5.11kohm (or 4. 464ohm).