M. Amer Iqbal Qureshi

Teach Yourself PIC Microcontrollers | www.electronicspk.com | 18 
to find out its contents. 
Memory components are exactly like that. Each memory address corresponds to one memory location. The 
content of any location becomes known by its addressing. Memory consists of all memory locations and 
addressing is nothing but selecting one of them. This means that, on one hand it is necessary to select the 
desired memory location, on the other hand it is necessary to wait for the contents of that location. In 
addition to read, memory also has to allow writing to these locations. There are several types of memory 
within the microcontroller: 
ROM memory (Read Only Memory) 
ROM memory is used to permanently save program being executed. Clearly, the size of a program that can 
be written depends on the size of this memory. Today’s microcontrollers commonly use 16-bit addressing, 
which means that they are able to address up to 64 Kb memory, i.e. 65535 locations. For the sake of 
illustration, if you are the beginner, your program will rarely exceed limit 
of several hundreds instructions. There are several types of ROM. 
Masked ROM. Microcontrollers containing this ROM are reserved for 
the great manufacturers. Program is loaded into the chip by the 
manufacturer. In case of large scale manufacture, the price is very low. 
Forget it... 
OTP ROM (One Time Programmable ROM). If the microcontroller 
contains this memory, you can download a program into the chip, but the 
process of program downloading is “one-way ticket”, meaning that it can 
be done only once. If you after downloading detect some error in a 
program, the only thing you can do is to correct it and download that 
program to another chip. 

tải về 3.06 Mb.

Chia sẻ với bạn bè của bạn: