assembly code for lcd display in stock

2023-01-03 12:32:11

We come across Liquid Crystal Display (LCD) displays everywhere around us. Computers, calculators, television sets, mobile phones, and digital watches use some kind of display to display the time.

An LCD screen is an electronic display module that uses liquid crystal to produce a visible image. The 16×2 LCD display is a very basic module commonly used in DIYs and circuits. The 16×2 translates a display of 16 characters per line in 2 such lines. In this LCD, each character is displayed in a 5×7 pixel matrix.

Contrast adjustment; the best way is to use a variable resistor such as a potentiometer. The output of the potentiometer is connected to this pin. Rotate the potentiometer knob forward and backward to adjust the LCD contrast.

Sends data to data pins when a high to low pulse is given; Extra voltage push is required to execute the instruction and EN(enable) signal is used for this purpose. Usually, we set en=0, when we want to execute the instruction we make it high en=1 for some milliseconds. After this we again make it ground that is, en=0.

A 16X2 LCD has two registers, namely, command and data. The register select is used to switch from one register to other. RS=0 for the command register, whereas RS=1 for the data register.

Command Register: The command register stores the command instructions given to the LCD. A command is an instruction given to an LCD to do a predefined task. Examples like:

Data Register: The data register stores the data to be displayed on the LCD. The data is the ASCII value of the character to be displayed on the LCD. When we send data to LCD, it goes to the data register and is processed there. When RS=1, the data register is selected.

Generating custom characters on LCD is not very hard. It requires knowledge about the custom-generated random access memory (CG-RAM) of the LCD and the LCD chip controller. Most LCDs contain a Hitachi HD4478 controller.

CG-RAM is the main component in making custom characters. It stores the custom characters once declared in the code. CG-RAM size is 64 bytes providing the option of creating eight characters at a time. Each character is eight bytes in size.

CG-RAM address starts from 0x40 (Hexadecimal) or 64 in decimal. We can generate custom characters at these addresses. Once we generate our characters at these addresses, we can print them by just sending commands to the LCD. Character addresses and printing commands are below.

LCD modules are very important in many Arduino-based embedded system designs to improve the user interface of the system. Interfacing with Arduino gives the programmer more freedom to customize the code easily. Any cost-effective Arduino board, a 16X2 character LCD display, jumper wires, and a breadboard are sufficient enough to build the circuit. The interfacing of Arduino to LCD display is below.

The combination of an LCD and Arduino yields several projects, the most simple one being LCD to display the LED brightness. All we need for this circuit is an LCD, Arduino, breadboard, a resistor, potentiometer, LED, and some jumper cables. The circuit connections are below.

assembly code for lcd display in stock

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assembly code for lcd display in stock

LCD connected to this controller will adjust itself to the memory map of this DDRAM controller; each location on the LCD will take 1 DDRAM address on the controller. Because we use 2 × 16 type LCD, the first line of the LCD will take the location of the 00H-0FH addresses and the second line will take the 40H-4FH addresses of the controller DDRAM; so neither the addresses of the 10H-27H on the first line or the addresses of the 50H-67H on the second line on DDRAM is used.

To be able to display a character on the first line of the LCD, we must provide written instructions (80h + DDRAM address where our character is to be displayed on the first line) in the Instruction Register-IR and then followed by writing the ASCII code of the character or address of the character stored on the CGROM or CGRAM on the LCD controller data register, as well as to display characters in the second row we must provide written instructions (C0H + DDRAM address where our character to be displayed on the second line) in the Instructions Register-IR and then followed by writing the ASCII code or address of the character on CGROM or CGRAM on the LCD controller data register.

As mentioned above, to display a character (ASCII) you want to show on the LCD, you need to send the ASCII code to the LCD controller data register-DR. For characters from CGROM and CGRAM we only need to send the address of the character where the character is stored; unlike the character of the ASCII code, we must write the ASCII code of the character we want to display on the LCD controller data register to display it. For special characters stored on CGRAM, one must first save the special character at the CGRAM address (prepared 64 addresses, namely addresses 0–63); A special character with a size of 5 × 8 (5 columns × 8 lines) requires eight consecutive addresses to store it, so the total special characters that can be saved or stored on the CGRAM addresses are only eight (8) characters. To be able to save a special character at the first CGRAM address we must send or write 40H instruction to the Instruction Register-IR followed by writing eight consecutive bytes of the data in the Data Register-DR to save the pattern/image of a special character that you want to display on the LCD [9, 10].

We can easily connect this LCD module (LCD + controller) with MCS51, and we do not need any additional electronic equipment as the interface between MCS51 and it; This is because this LCD works with the TTL logic level voltage—Transistor-Transistor Logic.

The voltage source of this display is +5 V connected to Pin 2 (VCC) and GND power supply connected to Pin 1 (VSS) and Pin 16 (GND); Pin 1 (VSS) and Pin 16 (GND) are combined together and connected to the GND of the power supply.

Pins 7–14 (8 Pins) of the display function as a channel to transmit either data or instruction with a channel width of 1 byte (D0-D7) between the display and MCS51. In Figure 6, it can be seen that each Pin connected to the data bus (D0-D7) of MCS51 in this case P0 (80h); P0.0-P0.7 MCS-51 connected to D0-D7 of the LCD.

Pins 4–6 are used to control the performance of the display. Pin 4 (Register Select-RS) is in charge of selecting one of the 2 display registers. If RS is given logic 0 then the selected register is the Instruction Register-IR, otherwise, if RS is given logic 1 then the selected register is the Data Register-DR. The implication of this selection is the meaning of the signal sent down through the data bus (D0-D7), if RS = 0, then the signal sent from the MCS-51 to the LCD is an instruction; usually used to configure the LCD, otherwise if RS = 1 then the data sent from the MCS-51 to the LCD (D0-D7) is the data (object or character) you want to display on the LCD. From Figure 6 Pin 4 (RS) is connected to Pin 16 (P3.6/W¯) of MCS-51 with the address (B6H).

Pin 5 (R/W¯)) of the LCD does not appear in Figure 6 is used for read/write operations. If Pin 5 is given logic 1, the operation is a read operation; reading the data from the LCD. Data will be copied from the LCD data register to MCS-51 via the data bus (D0-D7), namely Pins 7–14 of the LCD. Conversely, if Pin 5 is given a voltage with logical 0 then the operation is a write operation; the signal will be sent from the MCS51 to LCD through the LCD Pins (Pins 7–14); The signal sent can be in the form of data or instructions depending on the logic level input to the Register Select-RS Pin, as described above before if RS = 0 then the signal sent is an instruction, vice versa if the RS = 1 then the signal sent/written is the data you want to display. Usually, Pin 5 of the LCD is connected with the power supply GND, because we will never read data from the LCD data register, but only send instructions for the LCD work configuration or the data you want to display on the LCD.

Pin 6 of the LCD (EN¯) is a Pin used to enable the LCD. The LCD will be enabled with the entry of changes in the signal level from high (1) to low (0) on Pin 6. If Pin 6 gets the voltage of logic level either 1 or 0 then the LCD will be disabled; it will only be enabled when there is a change of the voltage level in Pin 6 from high logic level to low logic level for more than 1000 microseconds (1 millisecond), and we can send either instruction or data to processed during that enable time of Pin 6.

Pin 3 and Pin 15 are used to regulate the brightness of the BPL (Back Plane Light). As mentioned above before the LCD operates on the principle of continuing or inhibiting the light passing through it; instead of producing light by itself. The light source comes from LED behind this LCD called BPL. Light brightness from BPL can be set by using a potentiometer or a trimpot. From Figure 6 Pin 3 (VEE) is used to regulate the brightness of BPL (by changing the current that enters BPL by using a potentiometers/a trimpot). While Pin 15 (BPL) is a Pin used for the sink of BPL LED.

4RSRegister selector on the LCD, if RS = 0 then the selected register is an instruction register (the operation to be performed is a write operation/LCD configuration if Pin 5 (R/W¯) is given a logic 0), if RS = 1 then the selected register is a data register; if (R/W¯) = 0 then the operation performed is a data write operation to the LCD, otherwise if (R/W¯) = 1 then the operation performed is a read operation (data will be sent from the LCD to μC (microcontroller); it is usually used to read the busy bit/Busy Flag- BF of the LCD (bit 7/D7).

5(R/W¯)Sets the operating mode, logic 1 for reading operations and logic 0 for write operations, the information read from the LCD to μC is data, while information written to the LCD from μC can be data to be displayed or instructions used to configure the LCD. Usually, this Pin is connected to the GND of the power supply because we will never read data from the LCD but only write instructions to configure it or write data to the LCD register to be displayed.

6Enable¯The LCD is not active when Enable Pin is either 1 or 0 logic. The LCD will be active if there is a change from logic 1 to logic 0; information can be read or written at the time the change occurs.

assembly code for lcd display in stock

As I’m now delving into the world of computer engineering, I thought I’d take you along in a series of tutorials on programming the 8051 microcontroller using Assembly.

You can view my complete compilation of tutorials in this Github repository, and the code for this tutorial in this folder. Since I’m on break from university and don’t have access to the cool electronics from our lab, I’ll be using MultiSim to construct and program the microcontroller circuits. (You can download MultiSim on Windows with a free 7-day evaluation, extended to 45 days upon request.)

In this tutorial, we’ll interface the 8051 microcontroller with an LCD and display a blinking “Hello World” message. LCDs are widely used in electronics for displaying messages and data, so hopefully you can apply what you learn here to your other projects!

8051 microcontroller (MCU). Built with 40 pins, 4KB of ROM storage, 128 bytes of RAM storage, and 2 16-bit timers. It consists of four 8-bit input/output (I/O) ports, each with bidirectional ability, i.e., the ability to receive data as input or send data as output. We will use these ports to interface with the LCD, and primarily configure the ports to output data to the LCD.

LCD (Liquid Crystal Display). Electronic display module. Here we use a 16x2 LCD, which means there are two lines and each can display 16 characters. In this LCD, each character is displayed in a 5x7 pixel matrix.

Crystal. Generates clock pulses required for the synchronization of all internal operations. We connect it to pins XTAL1 and XTAL2 of the MCU. Here we use a frequency of 11MHz.

Bussed three-line resistor R1. Serves as pull-up resistor for port 0 pins (port 0 always requires external pull-up resistors). Here we use a resistance of 10K.

Now let’s dive a bit deeper and take a look inside the LCD to see what’s going on. Below is a block diagram of the 16x2 LCD, adapted from this source and the HD44780U (LCD-II) datasheet. It may be a bit overwhelming, but we’ll break it down.

Let’s start with a high-level overview of the 16 pins of LCD module. When looking at a 16x2 LCD, you’ll see something similar to the figures below. The first is a representation of an LCD in MultiSim, and the second is an actual HD44780 LCD. You’ll only see 14 pins on the MultiSim representation since the backlight pins aren’t necessary.

Command register. Stores command codes, such as those to clear the display, set the cursor, and read/write data. Some common commands are shown below, where the codes are given by the values of RS, RW, and D0-D7.

Display Data RAM (DDRAM). Stores display data represented in 8-bit character codes. Each address in DDRAM corresponds to a position on the LCD, shown in the figure below.

Character Generator ROM (CGROM). Stores all permanent character patterns, e.g., letters of the alphabet. It generates 5 × 7 dot (pixel) character patterns from the 8-bit character codes in DDRAM. (If you’re not using the cursor, it’s 5 x 8.) Note that the patterns are stored such that their memory address is equivalent to the ASCII code for that pattern.

Step 1: Define custom characterSend address of CGRAM as a command to LCD to store custom character in that location. The (hex) address of the first character you define would be 40, the second would be 48, etc.

Step 2: Display custom characterSet the LCD cursor where you want to display your custom character. E.g., if it’s the beginning of the first line, send command 80 (hex).

DDRAM stores the ASCII code of custom character. (In this case, since the ASCII code is in the range mapped to CGRAM, it uses the patterns stored there to generate the character display.)

Display controller matches ASCII code of custom character in DDRAM with the CGRAM address and displays it on the LCD according to the character pattern in CGRAM.

Busy flag (BF). Checks the internal status of the LCD. When the busy flag is 1, the LCD is still executing previously-received commands and the next instruction will not be accepted. When the busy flag is 0, the LCD is ready to accept new information and the next instruction can be written.

LCD driver. Receives low-level instructions and turns them into waveforms for controlling commons (COM) and segments (SEG), i.e., sub-units of the display. (For a good explanation of this, see this pdf).

Now that we have some background on the LCD structure, let’s discuss the control pins a bit more. Recall from the section on LCD pins that there are three signals that the microprocessor uses to control the data transfer with the LCD module. A summary of these pins and their functions is given in the table below.

RW (read/write). This pin switches between writing data to the LCD and reading data from the LCD by setting RW = 0 or RS = 1, respectively. (RW is usually set to 0 since only the “Get status” instruction requires it.)

E (enable). This pun functions as the command/data latching signal for the LCD. When E=0, the LCD does not care what is happening with RW, RS, and the data bus lines. When E = 1, the LCD is processing the incoming data. However, it is really an edge-triggered pin, in particular negative edge-triggered. This means the LCD will latch in whatever is on the Data Bits and process it on the falling edge of the E signal, i.e., transitioning from high (1) to low (0).

The diagram below shows the timing for the LCD write cycle, demonstrating the interactions between the RS, RW, E, and data (D0-D7) signals. All 8 data signals are grouped together in the bottom row as their individual values don’t matter in terms of influencing the events of the cycle.

At event 1, the RW signal is set low to indicate the LCD is ready to accept data, i.e., the MCU can write data. RS is set as high or low depending on whether the MCU is writing data or commands, respectively. The data signals are ignored during this period.

When the MCU has information to send to the LCD, it loads data to the Data Bits and then asserts the E signal high at event 2 to indicate the data is available to read.

At some point in the interval between events 2 and 4 (i.e., while E is high), the data byte is sent to the data lines and the LCD reads the data bus (event 3). The next region is labeled “valid data,” as this is the actual data that will be written to the LCD.

On the falling edge of the E signal at event 4, the LCD latches the data and the data is no longer available on the data bus. The data must not actually change until data hold time is met at event 5. The RS, RW, and data lines are all free to change to other values after event 5. The only thing that you can’t do is bring E high again too soon, as a certain time buffer is required between writes to the device.

If we put all that we’ve learned together about the LCD structure and programming the control pins, we end up with a diagram that looks as follows. (Note that this represents “write mode,” i.e., RW = 0, and only considers CGROM for simplicity.)

In Assembly, any sort of conditional jump instruction (e.g.,JNC, JNZ, DJNZ) can be used for implementing for loops, but most often we use DJNZ (decrement and jump if not zero). In this directive, the register is decremented; if not zero, it jumps to the target address referred to by the label.

The structure of a for loop in Assembly is shown in the code below. Prior to the start of the loop a register is loaded with the number of iterations you desire. Once we arrive at the end of the loop, we use theDJNZ directive, which combines the register decrement and the jump back to the beginning of the loop.

Similar to functions in high-level programming, subroutines are “self-contained” pieces of code in a program that accomplish specific sub-tasks. In a given program, we often need to perform a particular sub-task many times on different data values. Rather than repeat the same block of instructions every time, we can package the instructions in a subroutine to call upon when needed.

A subroutine is called by transferring control to its entry point, so when the subroutine completes its execution, it needs to return control back to the calling routine. Therefore, whenever we create a subroutine, we end it with aRET (return) instruction.

In assembly code, theEQU directive gives a symbolic name (label) to a numeric constant, a register-relative value or a PC-relative value. It is essentially the same as setting a variable in higher-level programming languages.

Since we will use the RS, RW, and E pins a lot, we’ll assign symbolic names to the corresponding pins we’ve chosen (which depend on the wiring of the MCU to the LCD). In the circuit schematic above, you can see that bits 0, 1, and 2 of port 0 on the MCU have been wired to the RS, RW, and E pins, respectively.

We’ll also letDATA_PORT correspond to port 2, as bits 0–7 of port 2 are connected to pins D0-D7 of the LCD. We’ll discuss the labels defined in lines 6–7 in the section on the byte-looping subroutine.

Let’s say that rather than using the symbolic namesRS, RW, and E, we used P0.0, P0.1, and P0.2 everywhere. The first disadvantage of this is that if you changed the wiring such that the LCD control pins were connected to port 2 instead of port 0, you would have to replace every instance of P0 with P2. In the case of using variables, you would only have to change the defined ports and bits in the EQU statements.

The second disadvantage is readability. Looking through the code, you might forget, for example, thatP0.0 corresponds to RS. Using (well-named) variables eliminates this problem.

In Assembly, theDB (define byte) directive defines a byte-size variable, which can be in decimal, binary, hex, or ASCII format. In the code below, hex format is indicated by appending an “H” to the numerical value, and ASCII format is indicated by placing characters in quotation marks. (The assembler will assign the ASCII code for the characters automatically.) DB followed by multiple operands simply defines a succession of bytes. Note that if the operand is a string, it is already a succession of bytes, as each character takes one byte in memory.

We can access the data we’ve defined by assigning labels before the directives. In other words, we create variables storing the data. Later, we’ll use these variables to load the data in the main program.

This, in effect, creates a lookup table at theINIT_CMND address. We can think of each command as an entry in a table, with 38H being the first entry, 0EH being the 2nd, and 06H being the 3rd. The value of each entry can be accessed via its index in the table, which we will see come into play when we write both commands and data to the LCD one byte at a time.

We define variablesLINE1 and LINE2, containing the commands to bring the cursor to lines 1 and 2 in a similar fashion. Finally, we define variables DATA1 and DATA2, containing the data to display on lines 1 and 2 of the LCD.

Proper delays between actions in embedded systems are important so that the system has time to execute instructions. In our system, there are two ways we can ensure the LCD staggers its internal operations:Using just delays

In practice, it is recommended to use the second method, as the delay produced is almost exactly the amount of time the LCD needs for processing. However, for our purposes, we’ll stick with the first method, which is simpler but serves the same purpose as long as the delay is appropriate. (For an example of method two, see this article.)

One classic way to make a delay is to use nested decrement loops. Every time the inner loop counts down to 0, then the next decrements, and so on. The code for the delay subroutine we’ll use is shown below.

We can calculate the length of the delay our subroutine produces by analyzing the number of machine cycles (MC) taken for each instruction and taking into account the number of repetitions. We’ll break down our calculations by the inner and outer loops.

Now, we can find the delay in microseconds (us). The machine cycle for a system of 11.0592 MHz is 1.085 us, so the total delay of our delay subroutine is:

As you can see in the excerpts of code above, setting each pin low or high is relatively straightforward. TheCLR directive is used for the former and the SETB directive is used for the latter.

Understanding the first step, loading the data port, was more difficult for me, so let’s dive a bit deeper. In both excerpts, lines 3–4 correspond to step 1. The values of A (the accumulator register) and the DPTR (data pointer) function as parameters passed to the subroutine, together addressing the desired byte of data or command in code space. Here’s how it works:The DPTR points to the start of the lookup table.

The GIF below shows a visual of this process. Here, DPTR points toDATA1 and A iterates through each character of the data. (As a side note, a great way to debug your code is to check the values of registers at various “checkpoints.”)

Since we write data and commands one byte at a time, we now define a subroutine that loops through each entry in a given lookup table (pointed to by DPTR) and sends it to the LCD.

Since Assembly is a low-level programming language, and thus cannot be object-oriented, we go about this using the following method:A for loop iterates for a number of times equal to the number of entries (bytes) in the table.

Going back to the section on assigning symbolic names, you’ll see that we assign register 0 (R0) the symbolic nameNBYTES, and use it for storing the number of bytes in a table of data or commands. We also assign register 1 (R1) the symbolic name BYTE_IDX, and use it for tracking the current iteration we’re at when looping through the bytes.

You’ll notice from the code excerpt below that we increment A indirectly via the value ofBYTE_IDX. We do this because the WRT_DATA and WRT_CMND subroutines make use A for storing table entries, and simply incrementing A would make the index have a large offset.

Now that we’ve defined all of our subroutines, we can create the main body of our program. We begin by initializing the LCD with the commands in theINIT_CMND table. We first point the DPTR to the table and then specify the number of bytes the table has — 3 since there are 3 commands. After these “parameters” are set, we can call the SEND_CMND_BYTES subroutine.

As described for the initialization process, every time we prepare to send commands or data to the LCD, we point the DPTR to the appropriate table and specify the number of entries.

Congratulations, we made it to the end! We can finally put everything together. Remember to check out my Github for the program and MultiSim files, as well as other 8051 projects!

assembly code for lcd display in stock

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assembly code for lcd display in stock

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assembly code for lcd display in stock

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