5P49V5907_17 Datasheet, PDF (5/30 Pages) Integrated Device Technology – Programmable Clock Generator

5P49V5907 DATASHEET
Crystal Input (XIN/REF)
The crystal used should be a fundamental mode quartz
crystal; overtone crystals should not be used.
A crystal manufacturer will calibrate its crystals to the nominal
frequency with a certain load capacitance value. When the
oscillator load capacitance matches the crystal load
capacitance, the oscillation frequency will be accurate. When
the oscillator load capacitance is lower than the crystal load
capacitance, the oscillation frequency will be higher than
nominal and vice versa so for an accurate oscillation
frequency you need to make sure to match the oscillator load
capacitance with the crystal load capacitance.
To set the oscillator load capacitance there are two tuning
capacitors in the IC, one at XIN and one at XOUT. They can
be adjusted independently but commonly the same value is
used for both capacitors. The value of each capacitor is
composed of a fixed capacitance amount plus a variable
capacitance amount set with the XTAL[5:0] register.
Adjustment of the crystal tuning capacitors allows for
maximum flexibility to accommodate crystals from various
manufacturers. The range of tuning capacitor values available
are in accordance with the following table.
XTAL[5:0] Tuning Capacitor Characteristics
Parameter
XTAL
Bits Step (pF) Min (pF)
6
0.5
9
Max (pF)
25
The capacitance at each crystal pin inside the chip starts at
9pF with setting 000000b and can be increased up to 25pF
with setting 111111b. The step per bit is 0.5pF.
You can write the following equation for this capacitance:
Ci = 9pF + 0.5pF × XTAL[5:0]
The PCB where the IC and the crystal will be assembled adds
some stray capacitance to each crystal pin and more
capacitance can be added to each crystal pin with additional
external capacitors.
You can write the following equations for the total capacitance
at each crystal pin:
C_XIN = Ci₁ + Cs₁ + Ce₁
C_XOUT = Ci₂ + Cs₂ + Ce₂
Ci₁ and Ci₂ are the internal, tunable capacitors. Cs₁ and Cs₂
are stray capacitances at each crystal pin and typical values
are between 1pF and 3pF.
Ce₁ and Ce₂ are additional external capacitors that can be
added to increase the crystal load capacitance beyond the
tuning range of the internal capacitors. However, increasing
the load capacitance reduces the oscillator gain so please
consult the factory when adding Ce₁ and/or Ce₂ to avoid
crystal startup issues. Ce₁ and Ce₂ can also be used to adjust
for unpredictable stray capacitance in the PCB.
The final load capacitance of the crystal:
CL = C_XIN × C_XOUT / (C_XIN + C_XOUT)
For most cases it is recommended to set the value for
capacitors the same at each crystal pin:
C_XIN = C_XOUT = Cx → CL = Cx / 2
The complete formula when the capacitance at both crystal
pins is the same:
CL = (9pF + 0.5pF × XTAL[5:0] + Cs + Ce) / 2
Example 1: The crystal load capacitance is specified as 8pF
and the stray capacitance at each crystal pin is Cs=1.5pF.
Assuming equal capacitance value at XIN and XOUT, the
equation is as follows:
8pF = (9pF + 0.5pF × XTAL[5:0] + 1.5pF) / 2 →
0.5pF × XTAL[5:0] = 5.5pF → XTAL[5:0] = 11 (decimal)
Example 2: The crystal load capacitance is specified as 12pF
and the stray capacitance Cs is unknown. Footprints for
external capacitors Ce are added and a worst case Cs of 5pF
is used. For now we use Cs + Ce = 5pF and the right value for
Ce can be determined later to make 5pF together with Cs.
12pF = (9pF + 0.5pF × XTAL[5:0] + 5pF) / 2 →
XTAL[5:0] = 20 (decimal)
MARCH 3, 2017
5
PROGRAMMABLE CLOCK GENERATOR