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Text Script for eLearning course “Oscillators”

Table of contents

Topic Slide Number
Title Slide 1

Introduction 2
Oscillator Modes 3
High Speed 4
Standard Crystal 5
Knowledge Check #1 6
Low Power 7
External RC 8
Internal RC 9
External Resistor 10
Knowledge Check # 2 11
Closing Slide 12

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Slide 1: Introduction

Microchip Technology welcomes you to the Oscillator module of the PICmicro
x14 microcontroller. This module will give you a basic overview, design tips and
troubleshooting for PIC MCU Oscillators and is intended for those new to PIC
MCUs or those requiring a refresher on PIC MCU Oscillators. It is recommended
you have previously taken or are familiar with the PIC MCU x14 architecture and
instruction set modules.

Slide 2: Introduction

The oscillator circuit in a PICmicro is what generates the system clock that is
required by the MCU to run. While oscillator design and usage is simple in
concept, selecting the wrong or unqualified oscillator for your application can lead
to difficulties. This presentation explores these issues and the common oscillator
modes and attributes so you will have a greater understanding of designing with
oscillators on PIC MCUs.

Slide 3: Oscillator Modes Overview

There are 4 common oscillator modes that are available on PICmicro devices;
HS, XT, LP and RC. These modes allow the user to create a clock by using
crystals, canned oscillator modules, resonators, an RC circuit or even a single
resistor. (add pictures of different clock sources) Some PICmicro devices also
have the option of an internal RC oscillator. As you would expect, each of these
modes has a different set of operating characteristics and constraints.

The oscillator mode is set by the state of the configuration bits, wh ich are non­volatile memory locations, and are written during device programming. These
bits are set within the programmer software or by using the _config directive in
your source code. Please note here that the config directive has 2 underscore
characters, not one.

It should be noted here that no matter what oscillator mode you choose to use,
is very important that your design is tested thoroughly to make sure it is stable.

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Slide 4: HS Mode

The first mode that we will talk about is the High Speed (HS) mode, which is
designed to be used with crystals and resonators above a frequency of 4Mhz.
However, this is only a guideline, the important thing to remember about HS
mode is that it provides the highest drive level available. The PICmicro oscillat or
design requires the use of a parallel cut crystal. This is true for HS, XT and LP
modes.

The diagram shows a typical circuit for use with a crystal or Ceramic resonator.
A series resistor Rs may also be required for AT strip cut crystals. You can find
information in the datasheet that gives typical capacitor values for a given
operating mode and crystal frequency used. This diagram shows a typical
configuration, but there are many different circuit configurations that can be used
for crystals and resonators, some of which are described in the device datasheet.

Keep in mind that crystals and resonators must be driven by a signal, and the
gain on this signal controls whether an oscillation will occur and how strong it will
be. Care must be taken not to underdrive or overdrive the crystal or resonator.
With crystals, too much drive will cause long-term damage, often causing a
failure in a matter of weeks. With too little drive, the product may not start up
correctly or may fail to oscillate under certain conditions.

With resonators, more drive is typically required than for crystals so you will find
that HS mode is commonly used for resonators.

Keep in mind that as the frequency increases, the drive level required tends to
increase, as well as the current consumption.

Slide 5: XT mode

The next mode we will discuss is the standard crystal mode or XT mode. This
mode produces a medium drive level and is designed to be used with crystals
and resonators from 1 to about 4 MHz. XT mode has moderate power
consumption since its drive level is lower than HS mode, and because a lower
clock speed is produced.

If your circuit is having problems starting up in this mode, you may want to
consider using HS mode which has a higher drive level.

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Slide 6: Knowledge Check # 1
Q: In general, using a higher frequency crystal oscillator will:

a) cause unstable oscillation
b) damage components

c) cause higher current consumption (correct answer)

d) require the use of HF mode

Slide 7: LP mode

The Low Power (LP) mode is designed for circuits where low power is a primary
concern. LP mode is engineered for use with a 32.768 kHz crystal, although it
can function at any frequency below 200 kHz. The 32Khz crystal is commonly
used for timing -sensitive applications since this frequency is easily divided down
to one that is suitable for timekeeping functions.

It is important to note that in order to produce accurate timing, the system should
be as stable as possible with respect to variables like temperature, voltage, etc.

Slide 8: EXTRC mode

There are two RC modes available--one for external components and one for
internal components.

The External RC mode (EXTRC) uses a resistor-capacitor network connected to
the OSC1 pin. When the device is configured for external RC mode, these
components are automatically driven to produce a frequency which will run the
PICmicro MCU. The frequency range for EXTRC mode is DC - 4MHz and the
power consumed will vary depending on the frequency used. RC mode is
designed for very low-cost applications and an RC circuit is typically more rugged
than a crystal or resonator.

It is important to note that RC mode will produce an inaccurate clock source. The
frequency will vary due to temperature, process variance in the device, types of
capacitors and resistors, etc. For these reasons, RC mode is not recommended
for timing sensitive applications such as RS-232 communication.

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Slide 9: intRC mode

As mentioned earlier in the presentation, some PICmicro devices offer the use of
an internal RC oscillator. For these devices, the Internal RC mode (IntRC)
functions much like the External RC mode that we just spoke about, except that
the resistor and capacitor are provided on the chip. The frequency range
available for the Internal RC mode typically varies by product from 31 kHz to 4
MHz.

PICmicro devices with the IntRC mode often contain an Oscillator Calibration
(OSCCAL) register, which is used by the factory to calibrate the internal oscillator
as accurately as possible.

Since no external components are needed, IntRC mode is the least expensive
oscillator available. Another advantage of this mode is that the OSC1 and OSC2
pins can often be used as general purpose I/O pins.

As with External RC mode, the IntRC mode should not be used for timing
sensitive applications such as RS-232 communication.

Slide 10: ER mode

The last mode is the External Resistor (ER) mode which requires only one
resistor to operate. The value of the resistor is what sets the frequency range for
the ER mode. For information on the frequency resulting from various resistor
values, refer to the device datasheet.

Similar to RC mode, ER mode has moderate power consumption and is very low
cost. As with the other RC modes, it is not recommended for timing sensitive
applications.

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Slide 11: Knowledge Check #2
Q: Matching question - The correct matches are shown here:

HS mode – Highest drive level available
RC mode – Has both external and internal modes
XT mode – Produces lower clock speeds than HS mode
LP mode – Ideal for timing sensitive applications

Slide 12: Closing Slide

This concludes the Oscillator module for the PICmicro MCU. We hope you now
have a basic understanding of PICmicro MCU oscillators. Remember that it is
very important that you test your oscillator design thoroughly to ensure it is
stable.

There are several application notes that you may find useful when designing with
oscillators. These are AN588: “PIC16/17 Oscillator Design Guide” and AN606:
“Low power Design using PIC16/17”

If you would like to give us your comments on this presentation or the Microchip
eLearning Program, click on the “Feedback” link on the left side of the screen.

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