PIC n’Mix
Mike Hibbett’s column for PIC project enlightenment and related topics
Part 3: PIC18F development board
W
e continue this month with
Flexible – the ability to select which
Simplifying soldering
the process of creating a development board for the PIC18F
processor. In this and the following article we are going to cover the design of
the board, ensuring it supports a number
of different externally connected technologies: GPS location, digital compass,
various display types, speech output, connectivity to a PC connection via USB, Wi-Fi
communication, interfacing with Amazon
Alexa, servomotor control and analogue
input. The focus for this article series will
be on making use of the processor, rather
than diving into the finer details of internal processor peripherals – a hands-on,
practical approach.
In the previous article we selected the processor we will be using – the PIC18F47K42
in a 40-pin DIL package. We chose that
package because it provides lots of I/O
signals yet is very easy to solder (via a
40-pin socket) making it easy to replace
if damaged.
There is a lot of work to do before we
jump in and start designing a PCB. Even
before drawing the schematic, we need to
be clear in our objectives for this board,
writing down some requirements, and
check if any of our items might clash with
each other and (hopefully) find solutions
to those issues. Let’s start by writing down
our key requirements:
Easy to assemble – no tiny surfacemount soldered parts. We want this to
be easily assembled, by any hobbyist
features are fitted at any one time,
without major re-work
Re-usable – easy to re-purpose to
different projects
PC serial communications interface
using an on-board UART-to-USB
converter chip, the MCP2221A-I/P
Header connector for an ESP-01 Wi-Fi
interface
Header for a Micro SD Media card
module
Several three-pin headers for servomotors
Header for a colour touchscreen LCD
FET power switches for external
device control
PICkit 4 header for programming/
debugging
Header pins for external devices with
power and I2C or SPI buses
Two configurable op amps, for analogue input signal conditioning
32kHz crystal for very low-current
operation
Plenty of analogue input and digital
I/O headers
Single status LED, and a power LED
Power input from a standard DC
power brick.
The first requirement is driven by a desire
to enable this board to be built by as many
hobbyists as possible, avoiding the use of
difficult-to-solder surface-mount parts. This
is easy to achieve with most components,
but the USB and SD Media connectors are
only available in hard-to-solder formats.
Thankfully, this has been recognised by
a number of electronics suppliers who
provide very simple and low-cost preassembled carrier boards, as shown in
Fig.1 and Fig.2. We will also use a Wi-Fi
module with a similar simple header
interface (shown in Fig.3). These little
boards enables us to avoid the difficulty
of soldering ICs that are only available in
surface-mount format. Another benefit of
providing these as plug-in features is that
they are optional – buy them only if you
want to use them.
On our PCB, USB connectivity to a PC
is provided by a special IC, the Microchip
MCP2221A-I/P. Our processor does not have
USB built in, and the choice to use an external IC rather than choose a processor with a
USB peripheral is deliberate – in our experience the USB software libraries provided
by Microchip (and other vendors) are very
complicated to use. The MCP2221A provides a simple USB-to-UART conversion,
so our processor we will connect the IC to
one of our serial ports, and the USB cable
will look to the PC like a standard serial
port. This will make software development
Fig.1. USB connector board.
Fig.2. SD-Media connector board.
Practical Electronics | January | 2021
That’s quite a list, but many of these will
be easy to implement, and simply remind
us to include certain components in the
schematic. Many requirements have a
consequence – adding a 32kHz crystal,
for example, means that two GPIO pins
become unavailable. Fortunately, however,
there are many GPIO pins, so that won’t
be an issue for us.
Fig.3. Wi-Fi interface board.
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comes pre-loaded with software that is
easy to use and communicates with our
processor over one of the UART interfaces
(we have two UARTs on our processor, so
simultaneous Wi-Fi and USB connectivity
will be possible.)
Power supply design
Fig.4. Plug-in option boards.
on both the PC and the processor simple.
Plus, the MCP2221A can run at 3.3V or 5V,
so it’s very flexible.
An SD Media card communicates over
an SPI interface, so we will route one of
the SPI buses to it. SD Media cards operate at a voltage of 3.3V maximum, so bear
that in mind if you are thinking of running
your board at 5V. An external adaptor to
level-shift the signals will be required if
you want to use an SD Media card for data
storage and run the board at 5V simultaneously. This will be true for the Wi-Fi
interface too.
The downside of using standard components in a design like this is that the
PCB will larger than the more usual Microchip development boards, which use
tiny surface-mount components. This is
not really a big issue – the board is for
development purposes, so easy soldering
and easy access to components is more
important than size.
Plug-in options
Although this will be a general-purpose
development board, we do have in mind
some external devices that we would like
to be able to connect to it. Fig.4 shows the
current selection from our lab – An LED
matrix, touch-input colour LCD screen,
GPS module and a bag of over 30 different random sensors that were purchased
cheaply on eBay. We will provide links
for the parts used in upcoming articles,
as we make use of them. Needless to say,
these will just be examples, you are free to
attach whatever you want to your board.
Wi-Fi interface
Choosing a Wi-Fi interface board presented some interesting challenges. The board
has to be cheap, easy to purchase, easy to
solder and easy to use. We settled on a
relatively old device, the ESP-01, which
is based on the ESP8266 processor. This is
such a popular design that it has become
available from dozens of different suppliers and is incredibly cheap – as low as £3
including delivery, if you do not mind the
long delivery times from Asia. The module
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With those thoughts behind us, it’s time to
make our first key design decisions – how
will we power the board, and how will we
direct the processor’s 36 I/O pins to onboard and external connections?
We’ll start with the power supply design.
So, first, we must decide at what voltage
to run the board: 3.3V or 5V? These two
common operating voltages have for years
been a bone of contention for us, too often
we have tried to pair a 5V external device
with a 3.3V processor development board
(servomotors with a Raspberry Pi for example) or a 3.3V module with a 5V processor
board (a GPS module with an Arduino,
for example.) It’s been such an issue over
the years that we have decided to design
a board that can operate at either voltage,
selected by a simple jumper.
The key ICs in our design have been
selected as ones that can operate at either
voltage, so we only need a power supply
design that can be switched between the
two voltages – and we can do that with
an LM317 regulator and a few resistors.
We will use a beefy TO220-style regulator, as powering some external devices,
particularly servo motors, may bring our
current-consumption requirements up
to around 500mA or so. Plus, we make a
mental note: leave space around the regulator for heatsinking, the LM317 is a linear
regulator, and will get warm at these higher
currents. We could have gone with a much
more efficient switching-regulator design,
but that would add complexity. We will
typically be powering this board from a
wall socket power supply, so power supply
efficiency is not a design requirement.
Studying the datasheet of the LM317
we spot that is has a requirement for a
minimum 3V input to output voltage differential, so to generate a 3.3V or 5V output
will require an input power supply running
at a minimum of 8V DC. 9V and 12V DC
power supplies are common and cheap,
so the regulator choice works well.
We will include a power supply isolation header pin, so 3.3V or 5V can be
supplied to the board directly, if desired.
This will enable the testing of low-power
designs, or allow the addition of a more
efficient power supply should you want
to use a battery.
A barrel jack will be accommodated to
support plugging in a standard DC power
brick. There are several different standards
for barrel jack connectors (different pin diameters) so we have to make a decision on
which to go with – we chose the 2.5mm
P I C 18 F 4 7 K 4 2
MC LR / R E3
1
4 0
R B 7
R A 0
2
39
R B 6
R A 1
3
38
R B 5
R A 2
4
37
R B 4
R A 3
5
36
R B 3
R A 4
6
35
R B 2
R A 5
7
34
R B 1
R E0
8
33
R B 0
R E1
9
32
VDD
R E2
10
31
VSS
VDD
11
30
R D7
VSS
12
29
R D6
R A 7
13
28
R D5
R A 6
14
27
R D4
R C 0
25
26
R C 7
R C 1
16
25
R C 6
R C 2
17
24
R C 5
R C 3
18
23
R C 4
R D0
19
22
R D3
R D1
20
21
R D2
Fig.5. PIC18F47K42 processor pin-out.
version, as that seemed to be the most
common size in our lab. Header pins and
solder pads will also be provided for power
input, as these cost nothing but add flexibility to the choice of power connection.
External header pins
We have left the most complex design decision until last. The processor, shown in
Fig.5, has 36 I/O pins – how will we connect these? Thankfully, the pins are highly
configurable; internal peripherals such as
SPI, I2C and UART interfaces are all able
to be programmed to appear on any pin
from a selection of the 36 pins available.
While this simplifies circuit design and
PCB layout, it does add complexity during
software design. With a general-purpose
development board design, where we make
all pins available, this task is very complicated – but that task can be simplified by
writing up a spreadsheet for the pins, indicating where each one will go.
The approach taken here is that multiple
headers are provided for generic SPI and I2C
devices, several servomotor headers (using
the standard header layout) and specific
headers for the Wi-Fi, USB and Micro SD
Media boards. Two FET-controlled power
output headers are provided, and then all
I/O pins will go to 0.1-inch pitch header
strips. Even the I/O signals that are optionally connected to other headers on
the board will be routed to the generic I/O
headers. Three I/O pins will not be used.
PORTE.3 is shared with the master reset
pin MCLR, which we will keep as a reset
pin. PORTC.0 and PORTC.1 are multiplexed with the external crystal inputs,
which will be taken up with our 32kHz
crystal, for ultra-low power operation.
To finish off, two op amps will be
implemented on the board, but left unconnected, with headers to allow user
choice of where they connect.
Practical Electronics | January | 2021
GP I O 6 ( UA R T 2)
GP I O 7 ( UA R T 2)
W i- F i
GP I O 4 ( UA R T 2)
US B
GP I O 18 ( P W M)
O p am p
S ervo
GP I O 5 ( UA R T 2)
GP I O 8
GP I O 9
O p am p
GP I O 10
GP I O 11
GP I O 19 ( P W M)
I 2C
S ervo
GP I O 29 ( I 2C 1)
P ower
input
GP I O 30 ( I 2C 1)
GP I O 12 ( S P I 1)
GP I O 20 ( P W M)
S ervo
GP I O 13 ( S P I 1)
Micro- S D
Media card
GP I O 12 ( S P I 1)
GP I O 14
GP I O 15
I 2C
A dditional
power
headers
GP I O 29 ( I 2C 1)
GP I O 30 ( I 2C 1)
GP I O 1
GP I O 16
GP I O 2
GP I O 12 ( S P I 1)
F ET
F ET
GP I O 3
S tatus
LED
GP I O 21
P O R T C 0
GP I O 22
P O R T C 1
32kH z
crystal
GP I O 12 ( S P I 1)
S P I
GP I O 13 ( S P I 1)
GP I O 14 ( S P I 1)
GP I O 13 ( S P I 1)
GP I O 14 ( S P I 1)
LC D
MC LR
GP I O 23
GP I O 24
GP I O 25 ( A DC )
GP I O 26 ( A DC )
S P I
GP I O 12 ( S P I 1)
P O R T B 7
GP I O 13 ( S P I 1)
P O R T B 6
Deb ug
GP I O 14 ( S P I 1)
GP I O 27 ( A DC )
GP I O 28 ( A DC )
Fig.6. High-level schematic, focusing on the I/O signals.
A standard PICkit 4 8-pin header will be
included for programming and debugging.
Working with the above specification, a
high-level circuit design focusing on the
I/O signals can be drawn, shown in Fig.6,
which shows all the required signals, without specifically naming them. From this
diagram we can now work through mapping processor pins to the diagram. This
is a bit tedious and is best done by writing down a simple spreadsheet listing the
pin numbers and their use, confirming for
each pin that a chosen peripheral can be
mapped to the desired pin. The resulting
pin mapping is shown in Fig.7.
High-level design – done!
Having worked through those requirements, and followed the thought process
discussed above, we have arrived at a
clear high-level design. At this stage, we
have no further questions to answer – the
schematic can be drawn up, and a board
layout created, with the risk of unexpected
issues cropping up significantly reduced.
Finishing touches
Before getting to the PCB layout we note
a few addition requirements that we
want to capture.
Four holes in the PCB corners will
be provided for the mounting of simple
feet. 3.4mm-diameter holes will be used
to allow the use of cheap M3 bolts to be
used, but there will be space for off-theshelf rubber feet.
Space will be left around the LM317
voltage regulator to allow for a small
heatsink, should your attachments require large currents.
The silk screen of the PCB will provide clear pin labels for each header
pin. This is an important requirement
and will increase the size of the PCB
slightly as space beside the header
pins will be needed to allow the text
to be visible.
Although it will not impact the PCB
design, it’s worth noting that all ICs (the
processor, the USB interface IC and the
op amp) will be fitting using sockets.
Although this adds a few pounds to
the cost, it will make the board easier
to repair should accidents happen. You
are of course free to ignore this advice!
That’s all the hard work completed –
now to the PCB design itself, which we
will pick up in the next article.
References
Micro SD-card holder
www.pololu.com/product/2597
Wi-Fi interface
www.addicore.com/ESP8266-ESP01-p/130.htm
USB Micro-B connector
www.pololu.com/product/2586
Fig.7. I/O pin mapping.
Practical Electronics | January | 2021
LED Display
www.ebay.co.uk/itm/191736670249
45
