Tutorial 08 for Arduino: SPI Interfaces Video Lecture | Arduino Programming: Step by Step (to become an Expert) - Electronics and Communication Engineering (ECE)

everybody Jeremy Blum here back with
episode 8 of my Arduino tutorial series
sponsored by element14 this we're going
to be talking about the SPI or serial
peripheral interface bus also known as
the spy bus in this episode I'll be
using it with an SPI controlled digital
potentiometer that allow us to vary the
input voltage to various LEDs that will
change their brightness this is just
another way of changing LED brightness
instead of using pulse width modulation
there's a lot of other devices that use
SPI also for another example go check
out the video of my reaction project
which is an audio-visual theremin that
I've been working on for some time it's
kind of on hold now so that I could work
on these video series actually but on
that's an example of where I'm using SPI
to control a large multicolor LED matrix
so you can go check that out too if you
want to see another implementation but
let's get started we'll start on working
to control these LEDs with an SPI
enabled chip let's talk quickly about
how the SPI bus actually works it's a
little bit different from the I squared
C bus that we talked about last week
similar to I squared C it communicates
synchronously which means you need to
have a shared clock signal between all
the Masters and the slaves in this
picture I have two slaves and one master
drawn the master in our case will be the
Arduino the master generates a clock
signal is shared by the SPI slaves
there's also a send and received line
unlike I squared C where there's only
one data line here our data can be
transmitted in both directions enabling
for a higher throughput MOSI or master
out slave in goes from the master to the
slave and miso master and slave out goes
out from the slaves to the master you'll
notice that these lines can be shared
and no pull-up resistors are needed
unlike the I squared C bus that's
because we have a unique chip select
line that allows us to figure out which
slave we're talking to you generally
have one chip select for each slave that
you want to control you pull that chip
select line low to enable controlling
that slave you shouldn't ever have more
than one chip select line pulled low at
a time it's also possible to daisy chain
these chip select lines together so you
only use one output pin from the arduino
but we won't discuss that here that
Apple
a little bit more advanced and requires
some more logic on both ends of the
circuit today's goal is to wire up a
digital potentiometer chip specifically
the ad 52 o4 from analog devices to
allow us to control the voltage input to
several LEDs using voltage dividers that
we'll set up using the digital
potentiometers let's start by looking at
the data sheet for this part all I had
to do is type in a d50 204 into Google
and it's the first result that comes up
it'll generally bring you to the analog
device page for the part this will give
you a brief description it's also
available in a six channel version we'll
be using a four channel version although
we'll only use three of the channels
will control three LEDs today you can
see an example another example of this
in my reaction project as well I also
use a 50 204 as a digital pot to control
the volume of some speakers all right we
have datasheet up let's scroll through
it we want to start by finding the pin
out this will give you some timing
information okay here's a pin out that's
the surface mount inversion alright we
want the dip socket version this is ours
so let's zoom in on this and this is
what we'll be using to make our
schematic so today because the circuit a
little bit more complex I've already
made a schematic in Eagle CAD if you're
not familiar with eagle cat it's a free
open source program for making
electrical schematics and I'll have the
files for this uploaded after the video
is done okay so let's take a look at
this chip there's 12 pins on each side
24 pins total here we can see our ground
our chips lick the line over it means
it's active low in other words pulling
it down is what activates it a couple of
lines here that will just basically pull
high to set them to the default States
you have to worry about them too much
but if you want descriptions
descriptions are available down below
here we can see SDI but that stands for
SPI data in so that's the same as our
master out slave inline and this is our
master in slave outline and then here we
can see the clock signal that our SPI
bus will be using and on the other side
these are all the inputs for our digital
potentiometers let's take a look at
digital potentiometer number three each
one has three pins
a B&W W stands for washer a will be
getting hooked up to five volts and B
down to ground the washer was basically
a washer that we can digitally change if
you remember the pod I showed you back
in an earlier tutorial we can digitally
change the value of that washer to set
this voltage divider to whatever we want
so if we bring a line out from the
washer we can get a special input
voltage into our led that's not five
volts and we can control it with the
microcontroller bringing in a voltage
less than five volts will make the LED
dimmer
all right so let's look at the schematic
that I've made for this here's the
schematic that I've put together for
this circuit you can see that all the
pins are in the same order that they
were on the datasheet I showed earlier
note that the bubbles here indicate
lines that are active low logic the
triangle indicates a clock line when
drawing a schematic you can see I have
ground connected to ground VSS is also
indicating ground PR VDD and shutdown
I'll go up to VCC also known as 5 volts
because we just want to keep those high
the whole time so it doesn't shut down
or anything like that and then on the
arduino pin 10 is is generally what you
use for chips left but you can obviously
use other pins if you have more than one
chip select that you need to control pin
10 is the first default though when you
use the SPI interface and Arduino pin 11
will always be your serial data into the
slave so master out slave in pin 13 will
be your clock line always and pin 12
will always be slave in or sorry slave
out data or into the master we won't
really be using pin 12 I'll hook it up
anyway but we're basically just sending
data from the master to the slave okay
on the right side we can see that we
have all of our LEDs hooked up here so
coming out of the wipers like I said
before we're just going to have our LEDs
going to ground note that you always
need to have a resistor in line with
your LEDs this time I chose 220 ohms I
could have chosen a lesser value and
made them a little bit brighter but 220
will work just fine and then they go to
ground on the other end as I said before
all the a values we'll be using 1 2 & 3
here will be going up to 5 volts and the
B's will be going down to ground
/ you'll notice I have them going
through a 10k resistor this is
essentially creating a larger value on
the bottom of the voltage divider which
will cut out the bottom half of the
range for the LEDs this is because LEDs
have a turn-on voltage a little bit
above two volts so what i'm doing here
since the potentiometer goes from zero
to 10k i'm basically cutting out the
bottom half of the digital potentiometer
range so that we only go from 2.5 volts
up to 5 volts otherwise the LEDs will be
spending most of their time being off
and you can adjust this value based on
your heli days and and things like that
all right so that's the circuit let's
see what it looks like wired up here we
can see the circuit all wired up right
in the middle is our ad 52 our 4 chip
make sure you note that the top of the
chip is where the little indent isn't it
on the left side we have everything
connected to power and ground where I
showed in the schematic earlier these
green wires go to the chip select and
the other communication wires for the
SPI communication bus to the Arduino all
those pin numbers are specified on
Arduino website on the right side we can
see the three yellow LEDs that I have
hooked up to washers 1 2 & 3 and then we
have both the 220 ohm resistors and the
10k resistors here so 6 resistors in
total two for each led the 10k goes to
ground from Part D of the washer and the
220 ohm goes through the led
ok let's write the program that we're
going to use to control our digital
potentiometer as always we're going to
include any libraries that we're going
to be using so we're going to write
include SPI H to include the SPI library
next up I've already written it in we
have to set our slave select pin to 10
because you can potentially have
multiple slaves lift pins you'll note I
don't have to set anywhere the pin the
other communication pins the three other
ones MOSI miso and clock pins because
those will be handled automatically by
the SPI library but this one you need a
set so set that when you set as an
output just as we always do and then
we'll initialize the SPI class in the
setup function the next thing I'm going
to do is write our new function for
controlling the LEDs
based on an input that we give it the
reason I'm doing this is because it
takes a couple lines to control them and
I just want to simplify it for us so we
can just say set this led to this amount
so let's call that function void set LED
and reg int level so here's a breakdown
of what that what that's doing first off
it's void because it's not returning
anything we've done it before and had it
returned instant stuff this time it's
not going to return anything this is
just going to execute something and
that's it the function is called set LED
and has two parameters both integers the
first one is reg for register and the
second one is level so there's four
registers on the on this digital
potentiometer that we're using and we're
using one two and three although it's
important to note that those are
register 0 1 & 2 when you're using
microcontrollers and things like that
most numbering generally starts with 0
so we'll pass it as 0 let's say if we
want to turn on LED 1 and then we'll set
the level of that anywhere from 0 to 255
this is an 8-bit control digital
potentiometer so we go 0 to 255 256 bits
of resolution okay I'll turn your 56
pieces of resolution it's 8 bit so in
here the first thing we need to do is
choose our slave select that we're going
to be working with now on spi slave
select is active low which means you
need to pull that pin low to decide that
you're going to write to that device so
we'll digital write the slave select for
the digital potentiometer low and while
that pins low anything that we
communicate on the SPI bus will only go
to that device so if we have multiple
things in the same line we can
communicate to all of them so spi dot
transfer is the function we'll use to
send our information the way this
digital potentiometer is are expecting
to receive information is it expects to
receive one byte with the register and
then one byte with the level you want to
set it to so the first thing we'll do is
send at the reg and then we'll do SPI
transfer and we'll send it the level
next now we finished transmitting to it
so we want to write the slave select pin
back to high
let's just how you do it okay so that's
that now we can use that in our loop and
for the loop I've decided what we'll do
is we'll have each led in sequence right
rise up to full brightness then fall
down back to off and then go the next
led and we'll do the same thing so let's
do that
to do this we're going to do two for
loops inside of each other which is
something we have not done yet like I
said in the previous episode four loops
are really awesome and extremely useful
and they become even more useful when
you nest them so let's look at how we're
going to do this the first thing we're
going to do is loop through the
registers which I'm going to do with
this outside for loop right here so what
this for loop is going to do is it's
going to start at register 0 and then do
register 0 1 & 2 and increment by 1 each
time so we'll have some other stuff go
inside of this for loop so what this
will do let's think about this as if
we're just controlling the first
register right now so the first time for
this for loop what do you want to do we
want the LED to go to up to full
brightness and then back down so we'll
make another for loop that increments
through the brightness levels we'll do
int J equals 50 I'm starting at 50
because if you started completely off it
takes a while for the LED to turn on it
doesn't you can't actually see any
brightness coming from the LED for a
little while so I'm going start at 50
instead of zero and we'll do J less than
or equal to 255 incrementing by 1 each
time okay um and so with each increment
we will set the LED that's why we made
this function remember to make it easier
to do this will set the LED I comma J so
if we're on loop 0 through this it'll
set LED on register 0 to this value
through this for loop so that'll
increment each led in secession and then
we'll just do it the way of mmm let's
say 20 so that doesn't happen
instantaneously we can't even see it and
now we want to have it scale back down
so now we're just going to do the same
thing in Reverse but let's have it hold
at the top for just a little bit and a
half second or so and now we're just
going to do the same for loop and
reverse
so we'll do int J we can use J again
because they're two separate things
um we'll start it at 255 this time and
we'll have it until J is greater than or
equal to 50 and we'll do J minus minus
because we're going down this time and
again we'll use our set led function
that we made with I comma J as the
arguments once again and do that delay
one more time okay that should do it
let's see how our SPI digital
potentiometer controls our LEDs remember
that this was washer one washer two and
washer three so register 0 1 & 2 you can
see that they're changing brightness in
order a nice slow increase in a slow
decrease back down it makes a nice
effect this would be a nice thing to use
for something like a nightlight and you
could put different color LEDs in it or
something like that and make a nice
little glowing effect so this is our
digital potentiometer controlled by SPI
in action what's happening is the
voltage levels are changing instead of
us changing it by PWM and that's
changing the brightness of the LED and
that's it
alright thanks for watching this episode
of my Arduino tutorial series as always
if you have any questions please feel
free to post them on youtube
element14.com or my blog next week we'll
start working with the xB wireless
communication modules so if you don't
already have an XP shield and an XP
transmitter slash receiver now it'd be
time to go out and grab those will be
communicating between two arduino z' to
arduino who knows
using those transmitters and receivers
so I will see you guys next week
thanks for watching thanks to element14
for helping me to sponsor this video
series they were kind enough to provide
a lot of the materials that I'll be
using to create these tutorials feel
free to go visit their website at
element14.com check out their community
which is a great place to talk to you
about electronics the Arduino and
basically anything else engineering
related and they also have a store where
you can buy a lot of the parts that
we'll be using in these videos