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How Did I write the I2C-LCD Library || Explained
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Jul 23, 2022
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0:01
[Music]
0:09
hello and welcome to controllers tech
0:13
as mentioned in the previous video i
0:15
have brought another video explaining
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how the i2c lcd library was written
0:21
this video is for all of you whoever
0:23
wants to understand the library
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irrespective of what mcu you are using
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this is the i2 c lcd library source file
0:32
that we are going to discuss today
0:34
before we jump into it let's see the
0:37
connection one more time
0:39
this is how the pcf 8574 is connected
0:42
with the lcd and as you can see it only
0:45
requires 2 wires to be connected to the
0:47
mcu
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here i am showing the connection with
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the stm32
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but it remains the same for whatever mcu
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you are using
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all you need to do is connect the clock
0:59
pin to the i2c clock and the data pin to
1:02
the i2c data
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the vcc must be 5 volts and ground to
1:07
ground
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this is how the actual pcf 8574 looks
1:12
like and as you can see this end must be
1:14
connected to the lcd vss and the other
1:17
end to the led minus pin
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by the way you can control the lcd
1:23
contrast using this potentiometer here
1:26
this picture actually represents how the
1:29
p-c-f is connected internally
1:32
this is very important to understand as
1:35
our library will be written based on
1:37
this connection diagram
1:39
as you can see only the four pins p4 to
1:43
p7 are connected to the lcd data pins
1:47
the pins p0 p1 and p2 are connected to
1:51
the rs rw and enable pins respectively
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although it's not shown here this pin p3
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is actually connected to the lcd
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backlight
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now that we have seen the connection
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let's check out the lcd 1602
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sheet also
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there are a lot of data sheets available
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but things mostly remain the same across
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them
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first of all we will check the
2:16
initialization part
2:19
as shown here this is the pattern to
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initialize the lcd in 4-bit mode
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we have seen in the connection diagram
2:27
that only four pins from the pcf are
2:30
actually connected to the data pins and
2:32
therefore we need to use the 4-bit mode
2:34
for the lcd
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as mentioned first we need to wait for
2:39
more than 15 milliseconds and then we
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need to send this
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notice here that the rs bit represents
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whether we are sending a command or the
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data in order to send the command this
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bit must be zero and to send the data it
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must be one
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also the rw bit indicates whether we are
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reading from the lcd or writing on it in
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order to write something we must keep
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this bit zero
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so here it means we are writing a
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command to the lcd
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basically the entire initialization
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sequence will be a set of commands and
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that's why the rs and rw bits will
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always be zero
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now we are only provided with the bits
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d4 to d7 so we will keep the lower four
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bits as zeros
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this will make the command as zero cross
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three zero
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then we need to wait for at least four
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point one milliseconds and send the same
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command again
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then wait for one hundred micro seconds
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and send the same command again
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here you can see in the library i have
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performed the mentioned operations in a
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similar way
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now we will send another command that is
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zero cross two zero and it will set the
4:10
interface to four bit length
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we are using the function lcd send
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command to send these commands to the
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display
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let's take a look at this function first
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as the name suggests the function can be
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used to send the commands to the lcd
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it takes the 8-bit command as the
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argument
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but since we are using the lcd in 4-bit
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mode we have to send this 8-bit command
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into two parts of 4-bit each
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so our first step would be to separate
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the upper half and lower half of this
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8-bit command
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we will perform the and operation with
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the 0 cross f0 and store this most
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significant half in the data u variable
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similarly we will shift the command by 4
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bits to the left position and then
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perform the and operation with the 0
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cross f 0 and store this least
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significant half in the data l variable
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now we need to add some other
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information to this command which will
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include the rs bit the rw bit and the
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backlight pin data
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we will start preparing the final
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commands and we will start by sending
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the most significant half first
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also remember that in order to send a
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command or data to the display we need
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to provide something called the strobe
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so after we send the data or the command
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we pull the enable pin high then pull it
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low
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this strobe is a kind of signal to the
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lcd that we have set the respective data
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and now it can process it
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here in this library i am sending the
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strobe along with the command itself
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so basically i am going to send the same
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command twice once with the enable pin
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high and the again with the enable pin
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low this means our p zero will be low to
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indicate that this is a command the p 1
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will be low to indicate that we are
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performing a right operation and p 2
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will be high to indicate that the enable
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bit is high and p 3 will be high so that
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the backlight remains on
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this will make the commander 0 cross 0c
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and we will add it with our original
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command and store it in the first
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position
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we will send the same command again but
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this time the enable pin will be low
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so the first two bytes we write to the
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lcd will have the command along with the
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strobe
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similarly we will store the least
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significant half in the other two bytes
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of the array
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finally we are ready to send this array
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of 4 bytes to the lcd
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now we can use the respective i2c
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function according to what controller
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you are using and send these 4 bytes to
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the lcd
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so basically we need the 4 bytes to send
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a single byte command
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similarly to send the data to the
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display we use this function lcd send
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data it also takes the data byte as the
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argument and then we edit that data
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before sending 4 bytes
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the only change we need to make is use
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the is bit as 1 to indicate that we are
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sending data this time
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other than that everything is similar to
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what we did while sending the command
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so now that we know how to send data and
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commands to the display let's continue
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with our initialization function
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we have reached up to this part where we
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set the display into the 4-bit mode
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the next few commands will be based on
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what kind of setup you want with the
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display
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notice here that now we have all the 8
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bytes for each command and they all will
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be used in one way or another
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here we have the function set command
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and to understand it we need to see the
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instruction set in detail
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here the function set command and it
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sets the display mode the number of rows
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and the font size
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dl controls the display mode 1 means
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8-bit mode and 0 means 4-bit mode
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n controls the number of rows one means
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two rows and zero means only one row
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and the f controls the font size you
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want to use
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i have already commented out everything
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i used in the library and you are free
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to modify these parameters according to
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your convenience
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the next command is the display switch
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command
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though it's not mentioned here you can
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check it by the position of the one in
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the command this command controls the
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display and the cursor
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we have to first turn off the display as
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mentioned in the initialization sequence
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the next command is to clear the display
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here it is mentioned in the first
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position
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note that all these commands take some
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time to execute and therefore we must
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provide a small delay after each command
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the next command is to set the entry
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mode
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here we can control the movement of the
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cursor along with the shift in the
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display
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i have set the cursor to increment and
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the display has no shift
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if you turn on the display shift you can
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control the shift direction by using
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this command later in the code
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finally we will turn on the display by
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using the same command again
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you can control the cursor and it's
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blinking also
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the pdf i am using doesn't seem to be
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much detailed so i would advise that you
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check a few other data sheets also
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this entire sequence will initialize our
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lcd
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now we can use the lcd data send
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function to send one byte of data to the
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display
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remember that you can only send data in
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the ascii format so you can send a
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single character using this function
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for example lcd sent data a or p or
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three
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we have another function here which can
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send the entire string to the display
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it will simply call the send data
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function as long as all the characters
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from the string are printed
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now let's talk about the lcd put cursor
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function
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this function sets the cursor at a
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specific location on the display
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so the lcd have ddram addresses and only
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certain addresses are available to print
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these characters
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i have covered this in the tutorial i
11:05
wrote for the esp32
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here are the ddram addresses available
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for the lcd 1602
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the top row address starts from 0 cross
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8 0 and we can display 16 characters in
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this
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the bottom row starts from 0 cross c 0
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and we can display another 16 characters
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in it
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so this function takes the row and
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column as the parameter
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and let's say if we want to print at the
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zeroth row and fifth column
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since the zeroth row starts from zero
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cross 8 0 it will add the column to this
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value
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we will get the address 0 cross 8 5 and
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we will send this as a command to the
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lcd indicating that we want to print at
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this address
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now if you send some data it will
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display at this particular location
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similarly if you give the input as first
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row and ninth column it will add the
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column that is nine to zero cross c zero
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this will result in the address zero
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cross c nine
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and then we can print at this particular
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location by sending the data next time
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these are all the functions available in
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the library and i hope you understood
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how i wrote them now let's also talk
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about the slave address of the pcf 8574
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device we are using
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you can see the address is defined here
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and i will explain why i am using this
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address
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let's check out the data sheet of the
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pcf-8574
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here are the addresses defined for
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pcf-8574
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or pcf-8574-a
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i have the
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pcf-8574 so i will explain its address
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the four most significant bits are 0 1 0
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0 and then the next 3 bits are a 0 a 1
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and a 2.
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these 3 bits are responsible for
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controlling the address of the device
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here are the pins a0 a1 and a2 and the
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pins next to them are ground
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right now the pins are high and if we
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solder them to ground the respective
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pins goes low
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the next bit we need to consider is the
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least significant bit which is the rw
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bit
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it is used along with the address to
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indicate whether we are performing the
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read or write to the device so by
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default the address is 0 cross 4 e if we
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consider the read write bit and 0 cross
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2 7 if the read write bit is not taken
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into account
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depending on whether your m c u supports
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7-bit address or 8-bit address you can
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define it accordingly
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so this is it for the video
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i hope you understood how the library
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was written
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the usage of the library has already
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been explained in the previous videos
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anyway i will leave the links to those
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videos in the description below
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leave comments in case of any doubt
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that's it for
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today keep watching and have a nice day
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ahead
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[Music]
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you
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