Introduction to Microcontrollers Lab Manual - Microchip

8.1 OBJECTIVE
8.2 PRE-LAB
Lab 8. External Memory
LABS
Investigate the MX PIC24F module memory resources and the general design issues
surrounding memory use in embedded systems.
8.2.1 Reference Material
• PIC24FJ256GB110 Family Data Sheet (DS39897)
• 16-Bit MCU and DSC Programmer’s Reference Manual (DS70157)
• MPLAB IDE User’s Guide (DS51519)
• PIC24F Family Reference Manual – Section 23. Serial Peripheral Interface (SPI)
(DS39699)
8.2.2 Memory Basics
At a basic level, memory circuits hold their contents for future accesses. Most memory
provides an interface to write information into the memory (i.e. store) and to read information
out of the memory (i.e. load). The write interface is not always available during
run time, e.g. One-Time-Program (OTP) memories.
The ability of a memory circuit to hold its contents when power is removed is one way
to categorize memory. The contents of volatile memory are erased when power to the
circuit is disconnected. Examples of this are the DDR memory of a desktop computer.
Nonvolatile memory holds its contents even when power is removed from the circuit.
Examples of this are a USB disk that contains NAND Flash memory.
8.2.3 Memory in Embedded Systems
In some ways, developing firmware for an embedded controller is not unlike writing
code for a desktop computer. Many aspects of the design process are similar: languages
can often be the same (i.e. “C”), the control loops are similar, the sequential
access is the same, and even multi-tasking and threads can be used on each. But
when it comes to memory, the equations are completely different.
A desktop computer has resources that are simply not feasible for embedded systems,
e.g. a large hard drive. As a result, most modern OSs assume that a large hard drive
is connected to the computer. By using a technique called “virtual memory”, a program
that is executing has access to an almost unlimited amount of memory. If the OS needs
more memory, then it will swap out little used areas of its volatile memory to the hard
drive. This then allows the OS to use that volatile memory space for further program
execution. If the memory that was previously swapped out is required, then the OS will
swap it back with another area of memory. As a result, most programmers can assume
that they have a virtually unlimited amount of RAM memory to use, and the OS takes
care of the details.
2011 Microchip Technology Inc. DS51963A-page 57