Tutorial Objectives

• Give a technical definition for the term microcontroller.
• Explain the major differences between a microcontroller and a microprocessor.
• Discuss the advantages of a similar microcontroller vs microprocessor product.

Microcontroller vs Microprocessor

What is a microcontroller?

A Microcontroller is basically a computer on a chip. It differs form normal desktop or laptop computers in that a microcontroller is an application specific computer that usually runs a single program performing dedicated task(s) while the the later two are general purpose computers that can run numerous programs depending on a users needs. A microcontroller contains on chip CPU, input/output interface, memory, clock, timer, and an assortment of of other peripherals.


A Microprocessor on the other hand is just a CPU one has to add externally memory, clock, input/output interfaces, timer and all other needed peripheral. This is the reason a microprocessor has so many pins.

1. Product smaller - A microcontroller product has many components on one chip and so is more compact. With a microprocessor product one have to add several other chips and so its bulky.
2. Cheaper - All the components on a microcontroller is manufacturer on a single chip and so is mush cheaper than having to manufacture several chips as is the case for a microprocessor system.

   Introduction to the Program Counter

   ---

   What is the program counter?

   The Program Counter (PC) is a register that is apart of all central processing unit (CPU) or microprocessor. All microcontrollers contains a microprocessor and thus has a program counter. The purpose of the program counter is to hold/store the address of the next instruction to be executed by the microcontroller's microprocessor.
   The size (width) of the program counter of a microcontroller is measured in bits and is directly related to the size of the microcontroller's program memory.
   
   

   Determining the size of the AVR Program Counter

   ---

   As mentioned above the purpose of the program counter (PC) is to hold the address of the next instruction to be executed by the CPU. Instructions are stores in the program memory of a microcontroller and thus the PC width is directly related to the size of the micorcontroller program memory. As an example lets determine the width of the program counter for the ATMega16 and ATMega32 AVR microcontrollers.
   
   
   The width(size) of the program counter (PC) is basically the smallest possibly number of bits necessary to address the microcontroller program memory based on its organisation. Lets say n is the width of the PC then n would be related to the size of the program memory by the following equation:
   
   
   2ⁿ = size of program memory
   n = log(size of program memory)/(log2)
   
   
   For the ATMega8515 microcontroller the program memory is 8k-Bytes organised as 4k-Words. As such the width of the ATMega8515 microcontroller Program Counter n is given by:
   2ⁿ = 4k = 4x1024
   n = log(4096)/log(2) = 12bits
   
   
   For the ATMega16 microcontroller the program memory is 16k-Bytes organised as 8k-Words. As such the width of the ATMega16 microcontroller Program Counter n is given by:
   2ⁿ = 8k = 8x1024
   n = log(8192)/log(2) = 13bits

Tutorial Objectives

• Give a technical definition for the term memory map.
• Draw the memory map of an AVR microcontroller.
• Extract the necessary information for a microcontroller's datasheet to create its memory map.
• Give a technical definition for the term stack.
• Give a technical definition for the term stack pointer.
• Explain the operation of the stack and stack pointer.
• Write assembly code to initialize an AVR stack pointer.

AVR Microcontrollers Memory Map

• 32 8-bits General Purpose
• 64 8-bits Input/Output Registers
• 512 8-bits SRAM space

• 8K byte Flash Memory
• Organized as 4K-16bits space

• 512 8-bits EEPROM space

1. Although this map includes the EEPROM as part of the memory map its is seen by some as a peripheral and not included as part of the map.
2. This memory map is basic. We could expand the General Purpose and Input/Output giving more details.
3. The external memory option could also be included in the memory map.
4. Each section can also be seen as a memory map in and of itself. Namely the Data Memory Map, theProgram Memory Map and the EEPROM Memory Map.

AVR Stack & Stack Pointer

---

What is a stack?

A stack is a consecutive block of data memory allocated by the programmer. This block of memory can be use both by the microcontroller internal control as well as the programmer to store data temporarily. The stack operates with a Last In First Out (LIFO) mechanism, i.e the last thing store on the stack is the first thing to be retrieved from the stack.

What is the Stack Pointer?

The stack pointer is basically a register or registers that holds either "the memory address of the last location on that stack where data was stored" or "the memory address of the next available location on the stack to store data." The definition of the stack pointer depends on the design of the microcontroller.

In AVR microcontrollers, such as the ATMega8515, ATMega16, ATTiny13, etc., the stack pointer holds the address on the next available location on the stack available to store data.

The AVR Stack Pointer

The AVR 8-bits microcontroller stack pointer can either consist of a single I/O register SPL (Stack Pointer Low) or two (2) I/O registers SPL and SPH (Stack Pointer High). The size of the stack pointer depends on the amount of data memory a microntroller contains. If the entire data memory can be addressed using 8-bits then the stack pointer is 8-bits wide i.e. SPL only, otherwise the stack pointer is consist of SPL and SPH.

Setting-up the AVR Stack

---

A programmer set up the stack on a microcontroller by loading the start address of the stack into the microcontroller stack pointer. The code below shows how to set up the stack in an AVR ATMega8515 microcontroller. Here the address being loaded is that of the last memory location in SRAM. This code could also be used for the ATMega16 , ATMega32 or any one of the AVR microcontrooler with a 16-bits stack pointer, you just of the replace the"m8515def.inc" include file with "m16def.inc" or "m32def.inc" respectively.


Notes:

1. RAMEND is a label that represents the address of the last memory location in SRAM. To use this label youMUST ensure that you include the definition header file for the specific microcontroller.
2. The functions low() and high() are use by the assembler to return the low byte and high byte respectively of a 16-bit word. Remember we are dealing with a 8-bit microcontroller which can only handle only 8-bits at a time. RAMEND is a 16-bit word and so we use the functions to split it.

.include "m8515def.inc"
 
  LDI R16, low(RAMEND)
  OUT SPL, R16
  LDI R16, high(RAMEND)
  OUT SPH, R16

AVR Microcontroller's Stack Operation

---

As mentioned earlier the stack can be use by both the microcontroller internal control and the programmer. The programmer accesses the stack using the PUSH and POP instructions. The PUSH instruction is use to store data on the stack while the POP instruction is used to retrieve data from the stack. The operation of the AVR stack will be discussed using the code that follows.


Notes:
Notice here the microcontroller being used in the code below is the Atmel ATTiny2313 AVR microcontroller. This AVR microcontroller's data memory can be addressed using 8-bits and therefore its stack pointer is defined only by the SPL.

/*
 *  Compatibly in both AVR Studio 5 & AVR Studio 6
 *
 *  Created: 10/15/2011 2:44:24 PM
 *  Author: AVR Tutorials
 */ 
 
.include "tn2313def.inc"
 
 ;Set up AVR ATTiny2313 stack
 LDI R16, RAMEND
 OUT SPL, R16
 
 LDI R16, 0x33
 LDI R17, 0x25
 LDI R18, 0x0A
 
 PUSH R16
 PUSH R17
 POP R17
 PUSH R18
 
 
end: RJMP end

Figure 1 - Initial Stack	Figure 2 - Stack After First PUSH
Figure 3 - Stack After Second PUSH	Figure 4 - Stack After POP
Figure 5 - Stack After Third PUSH