This is not a homework assignment; I am genuinely trying to grasp these concepts. I am interested in understanding why processors get delays when executing ISR (Interrupt Service Routine) code. I've heard that it's advisable to keep ISRs short and efficient. Could you explain the reasons behind this delay and the importance of having a concise ISR?"
Could you first explain why your question ends with a double quote character? To us, it looks like you copied it from somewhere, which indicates you are trying to minimize the effort you need to spend to obtain an answer to a question that asked
from you.
To understand these concepts better, buy a cheap, well-supported microcontroller development board with native USB (as that both opens up new options and experiments via USB HID, and reduces the amount of hardware you need – typically only a USB cable hub with a male end suitable for the development board), and start experimenting with the tutorials and examples. Currently, the Arduino world is easiest to handle.
I personally prefer
Teensy 4.0 with the
Teensyduino add-on to Arduino, for a number of reasons; one of them being that Teensy 4.0 is extremely powerful for a microcontroller. For learning purposes, something like a DigiSpark clone – ATtiny85 processor that uses the VUSB library to implement USB 1.0 using software only (1.5 Mbit/s), well suited for experiments with just a couple of I/O pins – may be more useful, because of their simpler and better documented hardware and software configuration and operation. You can buy these from eBay for under $2 apiece including shipping from China, and I bet there are many stores in India selling such also. My favourite 8-bit microcontroller is ATmega32u4, and it has a native USB interface and plenty of I/O, and is used in Arduino Leonardo and Arduino Pro Micro development boards, and it would be a very good starting point, but currently the development boards based on it are surprisingly expensive ($8 and up), because of the increased chip prices: even at JLCPCB assembly, a VQFN-44 ATmega32u4-MU costs $4.32. (You also need at least a crystal oscillator and related capacitors, some bypass capacitors, and some resistors on the USB D+ and D- lines, but they don't cost much.)
The most used Arduino microcontrollers are based on ATmage328 variants, which do not have native USB (but VUSB for very low-speed USB like USB HID is possible), and you also need an USB-UART adapter or a dedicated JTAG/ISP programmer to program those development boards.
Experimenting with these, for example using a single LED and a single tactile button to make a game where the point is to press the button one second after the LED was lit/turned off, with the LED then blinking in some pattern to tell how close you were, will give you a better intuitive understanding
that you need to ask the relevant questions. Even assuming you are perfectly honest about your intentions, you're struggling with the basic concepts,
because you have no personal experience or understanding to connect or slot the new information into. Playing with e.g. Arduino, and looking up the documentation to understand how the example code works, will give you exactly that connection or slotting possibility.
The same is with any university-level learning. Some people learn
by rote, i.e. memorize what their lecturer and book says, and consider that knowledge. It is not, it is only
data, or
potential knowledge; it is not
real. It only becomes real knowledge when you can
apply it to solve a problem. To do so, you must understand the problem, and how the various pieces can come together (and they almost always can in many different ways). Getting any kind of practical experience or intuitive grasp of the
problems, before learning their theoretical background, can make a massive difference. For me, if I can do that, I've found I can
integrate the basics of just about anything! That means, everything I learn this way, I can immediately use to solve problems. It is amazing how powerful this kind of learning is, so I do warmly recommend you learn to do it this way, too.
(However, not all humans learn the same way. I, for example, also need to use my hands, for example as a "temporary memory" for detailing things, because I otherwise will forget and not notice I forgot. Doing things with my hands seems to stimulate my mind, too. Others might need to talk about the thing – in programming, the Rubber Duck method works really well for some –, so if what works for me does not work for you, simply try the other ways, to find the ones that work for you best. It may even vary depending on the subject. The key is, find out how
you yourself learn best: not
memorize, but
integrate information into your understanding in such a way that you can use it to solve problems. Knowing what formula or approach to use to solve a problem is usually the key, but very often we end up choosing wrong, and asking questions like how to use that approach to solve a problem can be met with ridicule because those with experience see it as an obviously wrong approach. Thus, another key is to step backwards, and
see what assumptions you are making, and to try and describe the problem and your approach listing the
reasons for your choices, and the context in which you are trying to solve or understand the problem. This leads to the third key: the opinion of even a Nobel price winner is basically worthless, until you understand the reasons behind that opinion. What an authority states as a fact is
only a claim by an authority, until you can evaluate the basis: the reasons and reasoning behind that claim. This is why we haven't found anything that works better for science than critical peer review.)
Apologies for the wall of text.