PIC32MX130F128H-I/PF View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC32MX130F128H-I/PF

32-bit Microcontrollers (up to 512 KB Flash and 64 KB SRAM) with Audio/Graphics/Touch (HMI), CAN, USB, and Advanced Analog

Microchip Technology 

PIC32MX1XX/2XX/5XX 64/100-PIN FAMILY

2.7.1

CRYSTAL OSCILLATOR DESIGN

CONSIDERATION

The following examples are used to calculate the

Primary Oscillator loading capacitor values:

• CIN = PIC32_OSC2_Pin Capacitance = ~4-5 pF

• COUT = PIC32_OSC1_Pin Capacitance = ~4-5 pF

• C1 and C2 = XTAL manufacturing recommended

loading capacitance

• Estimated PCB stray capacitance, (i.e.,12 mm

length) = 2.5 pF

EXAMPLE 2-1: CRYSTAL LOAD CAPACITOR

CALCULATION

Crystal manufacturer recommended: C1 = C2 = 15 pF

Therefore:

CLOAD

= {( [CIN + C1] * [COUT + C2] ) / [CIN + C1 + C2 + COUT] }

+ estimated oscillator PCB stray capacitance

= {( [5 + 15][5 + 15] ) / [5 + 15 + 15 + 5] } + 2.5 pF

= {( [20][20]) / [40] } + 2.5

= 10 + 2.5 = 12.5 pF

Rounded to the nearest standard value or 13 pF in this example for

Primary Oscillator crystals “C1” and “C2”.

The following tips are used to increase oscillator gain,

(i.e., to increase peak-to-peak oscillator signal):

• Select a crystal with a lower “minimum” power drive

rating

• Select an crystal oscillator with a lower XTAL

manufacturing “ESR” rating.

• Add a parallel resistor across the crystal. The smaller

the resistor value the greater the gain. It is recom-

mended to stay in the range of 600k to 1M

• C1 and C2 values also affect the gain of the oscillator.

The lower the values, the higher the gain.

• C2/C1 ratio also affects gain. To increase the gain,

make C1 slightly smaller than C2, which will also help

start-up performance.

Note:

Do not add excessive gain such that the

oscillator signal is clipped, flat on top of

the sine wave. If so, you need to reduce

the gain or add a series resistor, RS, as

shown in circuit “C” in Figure 2-4. Failure

to do so will stress and age the crystal,

which can result in an early failure. Adjust

the gain to trim the max peak-to-peak to

~VDD-0.6V. When measuring the oscilla-

tor signal you must use a FET scope

probe or a probe with  1.5 pF or the

scope probe itself will unduly change the

gain and peak-to-peak levels.

2.7.1.1 Additional Microchip References

• AN588 “PICmicro® Microcontroller Oscillator

Design Guide”

• AN826 “Crystal Oscillator Basics and Crystal

Selection for rfPIC™ and PICmicro® Devices”

• AN849 “Basic PICmicro® Oscillator Design”

FIGURE 2-4:

PRIMARY CRYSTAL

OSCILLATOR CIRCUIT

RECOMMENDATIONS

Circuit A

Typical XT

(4-10 MHz)

1M

OSC2

OSC1

Circuit B

Typical HS

(10-25 MHz)

OSC2

OSC1

Circuit C

Typical XT/HS

(4-25 MHz)

Rs

1M

OSC2

OSC1

Circuit D

Not Recommended

1M

Rs

OSC2

OSC1

Circuit E

Not Recommended

Rs

1M

OSC2

OSC1

DS60001290E-page 28

 2014-2017 Microchip Technology Inc.

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