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EPE Tutorial Series
TEACH-IN 2006
Part Ten – PICs and Operational Amplifiers.
MIKE TOOLEY BA
Our Teach-In 2006 series provides a broad-based introduction to electronics for the complete newcomer. The
series also provides the more experienced reader with an opportunity to “brush up” on topics which may be
less familiar. This month we shall be looking at two very different integrated circuit devices that have both had
a huge impact on the world of electronics, the PIC microcontroller and the operational amplifier (op amp).
These deceptively simple devices have found their way into a wide variety of electronic applications and both
deserve a special mention in our Teach-In series.
PIC Microcontrollers
Last month, we introduced the notion of
the microcontroller as a device that can
form the basis of a stand-alone (or embedded) electronic application that can perform a variety of programmed functions. A
PIC is just such a microcontroller device
and it provides a flexible low-cost solution
to bridge the gap between single-chip computers and the use of large numbers of
discrete logic and other chips. PIC microcontrollers are being increasingly used in
“self-contained” applications involving
logic, timing and simple analogue-to-digital and digital-to-analogue conversion.
PIC microcontrollers first became popular more than a decade ago. Since then the
range and power of these devices has
increased dramatically. Nowadays, a PIC
device exists for almost any embedded
application, from small 6-pin devices ideal
for simple control applications, through to
powerful high-speed devices packed with
diverse I/O (input/output) features and
large amounts of program memory.
PICs are based on RISC (Reduced
Instruction Set Computer) architecture and,
as a consequence, they use a relatively small
number of instructions. In fact, some PIC
chips have as few as 33 instructions compared with some general-purpose microprocessors (such as the Z80 that we met last
month) that may have several hundred.
Because it is only necessary to remember a small number of commands, it is relatively easy to learn to program a PIC using
its own assembly language (note, however,
that it may take several instructions to
achieve on a PIC what can be done in a single instruction using a fully-fledged microprocessor). And, if you don’t like the idea
of having to learn assembly code programming you can make use of high-level languages such as C, BASIC or Flowcode (a
language based almost entirely on flowcharts). Program memory can be ROM,
42
PROM or Flash memory. Data memory, on
the other hand, must be read/write memory
(commonly known as RAM – see last
month).
As mentioned last month, mainstream
microprocessors (like the Z80) are often classified by the number of data bits that they can
manipulate (i.e. the “width” of the data bus).
This can be either 8-, 16-, 32-, 64- or 128-bits.
PIC microcontrollers, by contrast, are all
based on an 8-bit data bus and, because of
this, they can only operate on 8-bits of data at
a time (despite this, you will sometimes
encounter program instructions that reference
data in 12-, 14-, and 16-bit units). The
program memory of a PIC usually varies in
width from 12-bits to 16-bits.
PIC Families
In the early 1990s, PIC microcontrollers
were grouped in three families, often
referred to as “Base-Line”, “Mid-Range”,
and “High-End”. Unfortunately, many of
these devices are incompatible with the latest PIC development platforms and software tools. Despite this, it is usually
possible to locate a modern device that is
compatible with (and will generally outperform) one of these early devices.
Another important feature of modern
PIC devices is the use of electrically
erasable and programmable Flash memory
for program storage. These Flash memory
devices are often denoted by the use of the
letter “F” as part of the device coding (e.g.
PIC16F877A). Note, as we shall see later,
Flash devices are much easier to work with
for one-off prototyping because erasure and
reprogramming is greatly simplified.
Choosing a PIC Device
When choosing a PIC device for a particular project it is important to select a
device that is well supported, both in
terms of being a member of one of the current PIC families but also in relation to the
programming environment that you intend
to use for software development. It is also
important to ensure that the device incorporates all of the peripheral I/O facilities
that you will need. These features might
include:
Communication interfaces (such as
RS232/RS485, USB, etc)
Display peripheral interfaces (such as
LED or LCD drivers)
Capture/compare facilities
Pulse Width Modulators (PWM)
Counters/timers
Watchdog facilities
Analogue-to-digital (A/D) converters
Analogue comparators and operational
amplifiers
Brown-out detectors
Low-voltage detectors
Temperature sensors
Oscillators
Voltage references
Digital-to-analogue (D/A) converters.
The PIC16F84
The PIC16F84 is an excellent example
of a simple PIC device for use with a wide
variety of simple microcontroller projects.
The chip offers the following features:
35 single-word instructions
1K × 14-bit EEPROM (Flash) program
memory
68 × 8-bit general purpose static RAM
registers
15 × 8-bit special function hardware
registers
64 × 8-bit EEPROM data memory
1,000,000 data memory erase/write
cycles (typical)
Data retention > 40 years
5 data input/output pins (Port A)
8 data input/output pins (Port B)
25mA current sink maximum per pin
20mA current source per pin
8-bit timer/counter with pre-scaler
Power-on reset (POR)
Everyday Practical Electronics, August 2006