Make it with Micromite
Phil Boyce – hands on with the mighty PIC-powered, BASIC microcontroller
Part 43: Connecting more than one display to a PicoMite
O
K, so this month’s title
is probably not what you were
expecting, especially if you had
started building Part 1 of the Fingerprint
Reader. Now for a confession, I seem
to have somehow destroyed a second
fingerprint module, and hence I’ve had
to purchase yet another replacement.
This is all rather ironic bearing in mind
the first unit I received for free (since my
supplier had sent it in error in place of a
GPS module that I’d ordered – but said I
could keep it). So, while I await delivery
of what will be the third fingerprint
module, this month we’ll be discussing
an alternative topic, one that I hope you’ll
find useful: how to use multiple displays
with a PicoMite (or MicroMite). Why is
this of interest? Remember, by design,
MMBASIC’s firmware is only capable
of driving a single external display (the
specific type being configured with OPTION
LCDPANEL). So, how can we connect more
than one display to a PicoMite?
Examples of use
First, let’s acknowledge that there are some
restrictions as to what is achievable when
driving more than one display – but, once
these limitations are understood, it will
be possible to achieve some very creative
multi-display projects. For example, how
about adapting a project that uses multiple
7-segment LED modules and replacing each
one with a small LCD display module. This
would allow each ‘digit’ to offer endless
options in terms of colour and font. Or
how about building a data logger that has
multiple sensors, and providing each sensor
with a dedicated mini-display to show
relevant data. Another idea would be to
modify the Micromite Robot’s animated
eyes (that originally used two 8x8 LED
matrix modules) and replace them with
two coloured LCD displays; this would
allow much more detailed eye effects to
be animated.
52
Concept
n
Connect all display modules (effectively
The thought process when exploring this
topic was as follows. If MMBASIC can
send data to successfully drive a single
display, then what is preventing a second
identical display (connected to the exact
same PicoMite pins) from displaying
the exact same ‘image’? Providing the
displays are just receiving data from
the PicoMite (and not sending data back
to the PicoMite), then in theory you
could connect multiple identical display
modules to the PicoMite and they will all
display the exact same image when the
data is simultaneously received by them.
The reason we don’t want to concern
ourselves with data sent from a display
module back to the PicoMite is because
it makes the overall design much easier
(and also eliminates any potential shorts
that could occur between conflicting
logic levels).
Now for an important point: the
image on a display is only updated by
MMBASIC graphical commands, such
as TEXT, BOX, CIRCLE, PIXEL, LINE,
CLS, RBOX, POLYGON and so on. Once
the MMBASIC firmware has processed a
graphical command and the PicoMite has
sent the relevant data to the display(s),
the updated image on the display(s) will
remain in place until the next graphical
command is sent (or until power is
removed from the display modules).
Put another way, there is no need to
continually write data to the display(s)
to maintain the image.
Having the same image on all displays
isn’t very useful since ideally we will
want to control each display separately.
However, if each display also has some
form of ‘enable’ pin, then in theory, only
the displays that are enabled would
process the data sent out by the PicoMite.
So, theoretically, all we need to do for
multiple display control is the following
in terms of hardware:
in parallel) to the PicoMite in the usual
manner apart from the ‘enable’ pin of
each display
n
Connect each ‘enable’ pin from each
display to a dedicated PicoMite I/O pin.
Then, before using any graphical command,
enable only the display module(s) that we
want to receive that command by setting
the relevant I/O pins to the correct state.
Now that the concept has been covered,
we next need to understand any restrictions
when choosing and using multiple displays
with a PicoMite.
Restrictions
1. The first restriction is that all displays
used in a multi-display project must be
exactly the same type. This is because
the MMBASIC firmware can only be set
to a single type of display via the OPTION
LCDPANEL command.
2. The second restriction is the type of
interface that the identical displays use.
Referring to the PicoMite User Manual
(available for download from the January
2023 page of the PE website: https://
bit.ly/pe-downloads), you will see that
MMBASIC supports various LCD and
OLED displays that use either a serial
SPI, a serial I2C or a parallel interface.
However, the way that serial I2C and
parallel interfaces work means that the
data line(s) are used for both writing to and
reading from the display modules, and
hence there will likely be contradictory
logic levels present on the ‘common’ data
lines when connecting displays together
to the same pins. Only the SPI interface
has a dedicated signal for writing data to
the display. Thus, the second restriction
is that only displays that use a serial SPI
interface will work with the technique
we’re using here. To see this, think
about the serial SPI interface as having
three connections: SPI-Clk, SPI-Out and
Practical Electronics | January | 2023
Fig.1. Frequency counters typically uses
multiple 7-segment LED displays. This
one has 11 digits, resulting in a display
that is conveniently wide, but not too tall.
SPI-In. The PicoMite acts as the master
and generates the SPI-Clk signal while
transmitting any data that needs to be
sent to the displays via the PicoMite’s
SPI-Out pin (to the display’s SDA pin).
The PicoMite’s SPI-Out pin only sends
data to the display(s) and there is no risk
of any logic-level shorting. Note that the
SPI-Out pin has nothing to do with any
data that a display might want to send to
the PicoMite. Because we want to avoid
any potential logic-level shorts in our
application, we will not be receiving data
from the display modules, and hence
we will not be connecting anything to
the PicoMite’s SPI-In pin. If we were to
connect more than one display to the
single SPI-In pin, and if more than one
display module were to be enabled,
then logic-level shorting would likely
occur. Hence, we won’t read data from
any display.
3. Since we can’t read data from any of the
displays, certain graphical commands
(that require data to be read from the
display) will not work. For example,
the BLIT command that is used to copy
one area of the display to another is
unavailable; likewise, transparent text.
Understanding the above restrictions
means we can now experiment with the
concept; but first, let’s choose a multiscreen application.
Simple counter
number of digits to be displayed across
the screen. However, the proportions
of the screen would make it larger in
height than we really need; so instead,
let’s consider using multiple small LCD
displays for each digit.
We are not going to build a frequency
counter as such, but let’s take the idea
of building a simple 4-digit counter
comprising four displays; each one
displaying a single digit and all driven
by a single PicoMite. Four identical subdisplays means we can create an overall
display that is wide enough, but not
excessively high. Ideally. we want digits
that are about 25mm (1 inch) high, which
will be easy to read from a distance.
Now we know what we are trying to
achieve, let’s choose a suitable display.
ST7735S module
The ST7735S (see Fig.2) is one of our
favourite mini displays that is compatible
with the PicoMite, plus it meets all our
requirements:
n
SPI interface
n
‘Enable’ pin in the form of a ‘ChipSelect’
(CS) pin
n Compact size: 0.96-inch
Furthermore, it is an IPS display, which
means it provides a nice clear and bright
image that is viewable from most angles
(much better than an ordinary LCD display).
It is reasonably cheap – some well-known
online sites offer it at under £1.50 per unit,
although around £5 each is more common
from UK suppliers. So, this is the screen
we will use for our counter demo.
Fig.2 shows the ST7735S has just eight
connections, as follows:
1. BLK (Backlight) – connect this to any
I/O pin (defined as a parameter in the
OPTION LCDPANEL command) which
will then control the brightness of the
display via the BACKLIGHT command.
2. CS (ChipSelect) – the ‘Enable’ pin.
When low, the display is enabled.
Defined as a parameter in the OPTION
LCDPANEL command.
Fig.2. The ST7735S module is an 0.96inch IPS display module with a pixel
resolution of 160x80.
3. DC (Data/Command) – connect to
any available I/O pin. Defined as a
parameter in the OPTION LCDPANEL
command.
4. RES (Reset) – when low, the display is
reset. Connected to any available I/O
pin and defined as a parameter in the
OPTION LCDPANEL command.
5. SDA (SPI Data In) – connect to the
PicoMite’s SPI-Out pin (defined as
a parameter in the OPTION SYSTEM
SPI command).
6. SCL (SPI Clock) – connect to the
PicoMite’s SPI-Clk pin (defined as a
parameter in the OPTION SYSTEM
SPI command).
7. VCC (3-5V) – connected to 3.3V in our
application (can also be connected to
5V to give a brighter display image)
8. GND – connect to PicoMite’s GND pin.
A point to make here is that the MMBASIC
firmware drives pins 1-6 (once two
OPTIONs have been configured). The
remaining two pins simply provide power
to the display.
Demo 1
We will begin by connecting multiple
screens in parallel to check that we can
write to all displays simultaneously so
that they display the exact same image.
The easiest way to assemble this circuit
is to use a small piece of stripboard
Traditionally, multiple red
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
7-segment LED displays would
A
A
B
B
be used to build a counter display
C
C
D
D
since the displays are cheap and
E
E
are bright enough to read clearly,
F
F
G
G
especially when mounted behind
H
H
a red filter. Six, eight or even 10
I
BLK
GP11
J
CS
GP28
digits is not uncommon when used
K
DC
GP26
in something like a professional
L
RES
GP27
M
SDA
GP19
frequency counter (see Fig.1).
N
SCL
GP18
If we wanted to build a frequency
O
VCC
3V3
P
GND
GND
counter based on a PicoMite (a
Q
Q
great idea for a future article), we
R
R
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
could simply choose to implement a
single large LCD screen. If the large
8-way pin header
8-way sockets
screen is used in landscape mode,
then it would allow for a sufficient Fig.3. Four ST7735S modules are mounted in parallel on a piece of stripboard.
Practical Electronics | January | 2023
I
J
K
L
M
N
O
P
53
n
O PTION
LCDPANEL ST7735S,
orientation, DC, RESET, CS,
Backlight
by using: OPTION LCDPANEL ST7735S,
P, GP26, GP27, GP28, GP11
I/O GPx
10kΩ
GP28
CS
To display
CS
From
PicoMite
Fig.4. When the I/O pin is set high, the
display is disabled (because CS is high).
However, when the I/O pin is floating
(SETPIN GPx, OFF), then the CS signal
from the PicoMite can control the display’s
CS line. The 10kΩ resistor prevents device
damage when the I/O pin is set high and
CS is driven low by the PicoMite.
Once this is done, type any graphical
command to check that all displays behave
in the same way. For example, type CLS
RGB(GREEN) and observe what happens.
If all screens turn green, then you have
successfully completed the first step.
However, if things don’t work as expected,
visually check the eight connections to
the PicoMite, and also check the two
OPTIONs have been configured correctly
by typing OPTION LIST. If either OPTION
is incorrect, type OPTION RESET and
carefully re-enter both OPTIONs again.
Also check that there are no shorts between
adjacent tracks on the stripboard. Once you
see the correct response to any graphical
command, check that you can adjust the
brightness by typing BACKLIGHT 25.
This should noticeably dim the display.
Type BACKLIGHT 100 to return to full
brightness. If the display’s brightness is
unaffected by the BACKLIGHT command
then check the connection between pin
GP11 and the BLK pins on the displays.
Also check the last parameter in OPTION
LCDPANEL is indeed set to GP11.
since all we need to do is mount four
8-way sockets so that all displays are
connected to each other in parallel.
Refer to Fig.3 for how we prepared our
stripboard. The 8-way pin header on
the left is used with eight female-tofemale DuPont leads to connect it to the
PicoMite. Once everything is connected
and you have correctly inserted multiple
ST7735S display modules, connect the
PicoMite to your computer so that you
can configure MMBASIC to work with
the ST7735S. To do this, you just need
to set two OPTIONs as follows:
n
OPTION SYSTEM SPI CLKpin,
MOSIpin, MISOpin
by using: OPTION SYSTEM SPI GP18,
GP19, GP16
DIGIT1_DISABLE
GP0
1
40
VBUS
DIGIT2_DISABLE
GP1
2
39
VSYS
GND
3
38
GND
DIGIT3_DISABLE
GP2
4
37
3V3EN
DIGIT4_DISABLE
GP3
5
36
3V3
GP4
6
35
ADC REF
LCD_BACKLIGHT
GP3
GP11
BLK
GP2
10kΩ
GP28
Digit 4
GP26
DC
GP27
RES
GP19
SDA
GP18
SCL
GP28
33
GND
GP6
9
32
GP27
LCD_RES
GP7
10
31
GP26
LCD_DC
GP8
11
30
RUN
GP9
12
29
GP22
GND
13
28
GND
GP10
14
27
GP21
GP11
15
26
GP20
GP12
16
25
GP19
SPI_DATA
GP13
17
24
GP18
SPI_CLOCK
GND
18
23
GND
GP14
19
22
GP17
GP15
20
21
GP16
GP26
DC
GP27
RES
GP19
SDA
GP18
SCL
3V3
VCC
3V3
GND
GND
GND
GP28
Digit 3
LCD_CS
34
8
GP1
GP11
CS
10kΩ
3.3V
7
BLK
BLK
GP0
GP11
GP26
DC
GP27
RES
GP19
SDA
GP18
SCL
VCC
3V3
VCC
GND
GND
GND
GP26
DC
GP27
RES
GP19
SDA
GP18
SCL
VCC
3V3
GND
GND
GP28
Digit 2
BLK
CS
CS
10kΩ
Currently, the CS pin from the PicoMite
drives each CS pin simultaneously on
all four displays (and that is the reason
they all display the same image). We need
a way of holding any display’s CS pin
high whenever we want to ‘disable’ those
displays from responding to the data that
is being sent from the PicoMite (from the
PicoMite’s SPI-Out pin simultaneously to
all the SDA pins). By using a dedicated
I/O pin, we can hold an individual
display’s CS pin high, and then that
particular display won’t respond to any
SPI data that it receives. But we also
need a way of ‘releasing’ the logic-high
from the display’s CS pin whenever we
want the display to respond to the SPI
data. Refer to Fig.4 to see how we can
achieve this with a single I/O pin (GP0),
and a single 10kΩ resistor to control one
display. This circuit is repeated four
times, one for each display’s CS pin – in
other words, we will need four I/O pins,
and four 10kΩ resistors for
our four-digit counter.
GND
GP5
GP11
Modifying the CS signal
Display operation
GND
CS
The next step is to modify the stripboard
so that we only control displays that are
‘enabled’. Before we do this, let’s first
explore how to modify the CS pin so that
we can use a dedicated I/O pin (one per
display) to effectively enable (or disable)
the display.
10kΩ
GP28
Digit 1
Fig.5. Circuit diagram showing how the four ST7735S displays are connected to a single PicoMite.
54
With the I/O pin (eg, GP0)
in a floating state (SETPIN
GP0,OFF), the display’s CS
pin is controlled by the CS
output from the PicoMite.
In other words, the display
behaves normally. Note that
the 10kΩ resistor has no
impact on the logic level
received on the display’s
CS pin. However, if we now
set GP0 to a high logic level
(with SETPIN GP0,DOUT
and PIN(GP0)=1) then the
CS pin for that display is
held high and hence the
display is disabled and will
ignore any data received
on its SDA pin. In this
scenario, the 10kΩ resistor
ensures that the CS signal
BLK from the PicoMite is not
CS shorted by the logic high
DC from the GP0.
RES
In summary, with GP0
SDA floating, the display is
SCL enabled (and will respond
VCC to data), but with GP0 set
GND high, the display will be
disabled (and ignore any
data) – it’s that simple!
Fig.5 shows an overview
Practical Electronics | January | 2023
5-way pin header
enabled/disabled at any one time,
producing a flexible solution for
driving multiple displays from a
single PicoMite.
1) R1-R4 are 10kΩ
2) Four track breaks on row J at column 4, 14, 24, 34
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
A
B
A
B
C
D
E
F
G
GP1
GP0
DIGIT1
DIGIT2
DIGIT3
DIGIT4
CS
GP2
C
D
E
F
G
Challenge
Having seen how easy it is to
write to each individual display
I
module, how about challenging
BLK
I
GP11
CS
J
J
yourself to see if you can adapt the
K
DC
K
GP26
L
L
RES
GP27
Demo2 counter into displaying a
M
SDA
M
GP19
simple clock (hours and minutes
N
SCL
N
GP18
O
VCC
O
3V3
from the inbuilt TIME$ string
P
GND
P
GND
variable). Alternatively, connect
Q
Q
R
R
four temperature sensors to the
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
PicoMite, and then map each
sensor to its own display. And if
8-way pin header
8-way sockets
you’re feeling ambitious, why not
Fig.6. The modifications to the stripboard for individual control of each ST7735S display.
have a go at creating some kind
of game. Whatever your creative
juices come up with, do send us an email if you end up
of all the required pin connections that we’ll be using in
building something interesting.
our simple counter.
GP3
GP28
H
R
4
R
3
R
2
H
R
1
Modifications to stripboard
Next time
See Fig.6 for the changes that are required for the stripboard,
comprising the following:
n
Make four track cuts
n
Add a five-way pin header
n
Insert four wire links
n
Add four 10kΩ resistors.
Fingers crossed – the replacement fingerprint module will
arrive soon to complete Part 2 of the Fingerprint Reader project.
Until then, stay safe, and have FUN!
Once the CS signal modifications have been made on the
stripboard, and everything has been connected, you should
test that everything works correctly by running the Demo2.txt
program (available for download from January 2023 page of the
PE website). Install the program into your PicoMite and observe
what happens when you RUN the program. If all is working,
you will see a counter like that shown in Fig.7. Do makes sure
that you look at the code to see exactly how the DISABLE and
ENABLE subroutines control the CS line, as outlined in Fig.4
(and described above in the section Modifying the CS signal).
In essence, before issuing any graphics command, ensure
you enable each display that you want to respond, and
disable all the others. Note that one, some or all can be
Fig.7. Our simple 4-digit
counter comprises
four ST7735S display
modules assembled on
stripboard – all controlled
by a single PicoMite.
Practical Electronics | January | 2023
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Before modifying the stripboard, remove all the displays to
avoid accidental damage. Once the changes have been made,
perform a visual check to ensure there are no track shorts and
the four track cuts are made correctly. Once all looks good,
re-insert the displays, and connect the PicoMite to the two
headers, as shown in Fig.6 (left-hand side).
Questions? Please email Phil at:
contactus<at>micromite.org
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