Introduction:

For this project, I created an LCD clock. This project took me 5 weeks to complete. The code for the LCD clock was written in C on MIDE-51 and, as always, I used Tera Term, along with the LCD screen, to display the results of the code. 

In creating this project, I technically combined two previous projects (system integration), the Keypad project (C language) and the Clock project (C language). My goal was to, instead of typing in a desired time using a computer keyboard, use the keypad (attached to the circuit board) to enter the numbers.

Assembly vs. C

How much coding time can you save if you change from assembly language to C language in a microprocessor application?

To write the code for our clock project in both C and assembly, I spent a total of 3 weeks working on it. It took me a lot longer to write the code in Assembly than in C. 

A few reasons why this happened:

In C language, you only need to write one line to finish a standard IO interface, such as printf(). Lines like these can simplify the code. For example, writing printf() is much easier than calling getchar and putchar all the time. 

In C language, it's easier to write the code, but the code length is bigger and the program is slow.

In C language, variable memory location can be done by C compiler, assembly can not do so.

C does not need to assign labels in the programming. The standard loops (e.g. for(), while(), if() ) already embed the labels automatically. 

In Assembly, it's harder to write the code, but the code length is short and the program is fast. 

In Assembly, you have to manually plan out the memory location of each variable in the code. This can get tiring and confusing if you're working on a large-scale project.

You don't need to write semicolons after each line, which is required in C. You also don't need any parenthesis. 

If your code is complicated, the more advantaged you will be if you use C language. Our count-up was a simple application; therefore, the advantages of using C are not significant when writing the code. Our clock project was a complicated project with assembly language, after we change to C, it is much easier.

In conclusion, it depends on your code. If your code is complicated, you should consider using C language. If the code is simple, you can use either C or Assembly. For our code, using C was easier since we only used 1.5 hours to finish it. 

What is an interrupt?

Using an interruption in a microprocessor means that you are creating a temporary halt of the code in order for the program to go and execute something else. You can think of it as a real-life scenario. You're cooking dinner and all of a sudden, you get a phone call. The phone call is the interruption. After you're done with the phone call, you go back to cooking dinner. This is just like the scenarios in a microprocessor. 

Why do we need an interrupt? 

The purpose of the interrupt system is to improve the efficiency of the microprocessor when multi-event exists in an application. The interrupt system is important because some code executions are urgent, and need to be top priority. The user would have to wait for an important part of the code to be executed. 

How does an interrupt work?

In a microprocessor, the machine will be running the main code in an endless loop. If an interrupt occurs, the main code will halt and the microprocessor will start to run a code, called ISR - interrupt service routine, from a  code memory location called interrupt vector. Normally, the ISR is a short code and needs a very short amount of time. After ISR is finished the microprocessor will return back to the main code where it was interrupted and continue to execute the main code. 

For example, we can look above this text. The main code is running until it hits "lcall display". It jumps to Display, executes the code, and returns back to the main code. 

In summation, interruptions are crucial to the microprocessor. 

What I Learned: 

1. From this project, I learned the concept of system integration. During integration, some problems in the submodules were found and had to be fixed. Other problems could occur when changing from the Tera Term standard terminal to a dedicated hardware LCD and keypad. In order for the code to work smoothly, these problems needed to be fixed. 

2. Debugging:  I saw several problems after putting these 3 modules together. One problem was that the screen kept mixing the displayed characters with the new printed digit, making the display appear extremely confusing. In order to solve this, I created a clearscreen() to clear all of the unused characters. Another mistake was that I saw 2 unidentified characters after the clock display on the LCD screen. After further debugging, it was revealed that the problem was the incorrect expression of \n and \r (I had written \n as "/n") 🤦

Another example was the incorrect coding of the 3 necessary delays (essential in order for the code to work). I ended up forgetting to write one delay and having to recode another delay that was not updated in the past. 

3.  Changing the name of the function, especially when they use same name in the submodules. 

I had a delay() function in LCD, keypad, and clock. They had different time constants: 1ms, 20ms, and 1s. When I merged the 3 modules to LCD clock project, they were very confusing, so I had to rename them.