PIC16 Microcontrollers, Unit 4, Ch 2.3-2.4; Memory, Stack, and SFRs

Summary

In this lecture, Clint Halstead introduces microprocessors, focusing on program memory, the stack, data memory, and special function registers using the book 'Designing Embedded Systems with PIC Microcontrollers' by Tim Wilmshurst. He explains flash memory, program counters, the stack's LIFO structure, and their roles in executing and storing code. Halstead also covers SRAM, special function registers, and their use in interfacing with peripherals. He concludes by discussing the configuration word, its significance in setting microcontroller features, and its protection mechanisms against code theft.

Takeaways

• 📚 The lecture is an introduction to microprocessors, specifically using 'Designing Embedded Systems with PIC Microcontrollers: Principles and Applications, Second Edition' by Tim.
• 💾 The program memory is composed of non-volatile flash memory, which retains the program code even when power is removed from the chip.
• 🔢 The program memory space ranges from 0 to 3FF, providing approximately 1000 memory locations for storing code.
• 👉 The program counter is essential as it points to the next instruction in the program memory and starts at address zero upon power-up or reset.
• 📚 Section 2.3 and 2.4 of the book are being discussed, which cover program memory, stack, data memory, and special function registers.
• 🔠 The stack is a part of SRAM and operates on a LIFO (Last In, First Out) basis, crucial for function calls and preserving the state of the program counter.
• 🔄 The stack facilitates the 'push' and 'pop' operations, which save and restore the program counter's state during function calls.
• 🔩 Data memory is stored in SRAM, which is volatile and loses its values when power is removed, typically used for values set during program execution.
• 🌐 Special function registers are used for interfacing with peripherals, controlling aspects such as timers, port directions, and EEPROM memory.
• 🛠 The configuration word, found in program memory, determines certain operating features of the microcontroller and can be set during the programming process.
• 🔒 Code protection bits within the configuration word can prevent unauthorized reading of the microcontroller's program memory, protecting intellectual property.
• ⚙️ Oscillator configuration is vital as it dictates how the microcontroller operates, with options for internal (RC) and external oscillators.

Q & A

What is the primary function of program memory in a microcontroller?

The primary function of program memory in a microcontroller is to store the code, including assembler language code and C code, which the microcontroller executes.

Why is program memory often implemented using flash memory?

Program memory is often implemented using flash memory because it is non-volatile, meaning it retains the stored values even when the power is removed from the chip.

What does the program counter in a microcontroller do?

The program counter in a microcontroller points to the next instruction to be executed. It is set to zero upon power-up or reset, indicating the starting point of the code execution.

What is the significance of the stack in a microcontroller?

The stack in a microcontroller is used to store the return addresses during function calls, allowing the program to resume execution at the correct location after the function completes. It operates on a LIFO (Last In, First Out) principle.

How does the stack assist during function calls in a program?

During function calls, the current value of the program counter is pushed onto the stack to remember the current location in the code. After the function executes, the program counter is popped with the stored address, resuming the code execution from where it left off.

What is the role of the special function registers in data memory?

Special function registers in data memory control various aspects of the microcontroller's operation, such as timers, the program counter, status, file select for indirect addressing, and port control.

Why is SRAM considered volatile memory?

SRAM is considered volatile memory because it loses its stored values when power is removed from the chip, unlike non-volatile memory which retains values after power loss.

What is the purpose of the TRISC register in a microcontroller?

The TRISC register controls the direction of Port C on a microcontroller, determining whether the port operates as an input or output.

How does the configuration word in a microcontroller affect its operation?

The configuration word in a microcontroller determines certain operating features, such as code protection, power-up configuration, watchdog timer settings, and oscillator selection. It is set during the programming process and cannot be changed during normal operation.

What is the purpose of code protection bits in the configuration word?

Code protection bits in the configuration word prevent unauthorized reading of the microcontroller's program memory, protecting the intellectual property of the software developed for the microcontroller.

Why is it important to check the watchdog timer and oscillator settings when troubleshooting a microcontroller?

It is important to check the watchdog timer and oscillator settings when troubleshooting a microcontroller because incorrect settings can prevent the microcontroller from operating correctly. The watchdog timer may need to be disabled, and the oscillator must be correctly configured for the microcontroller to function.

Outlines

00:00

📘 Program Memory and Stack Overview

This paragraph introduces the concept of program memory in microprocessors, specifically focusing on the PIC microcontroller. It explains that program memory is composed of non-volatile flash memory, which retains the code even when power is removed. The memory is addressed from 0 to 3FF, providing approximately a thousand memory locations for code storage. The paragraph also introduces the program counter, which points to the next instruction in the program memory, and the stack, which is likened to a LIFO (Last In, First Out) structure used for function calls. The program counter's role in starting at address zero and the importance of the interrupt service routine starting at address 004 are also highlighted.

05:02

🔧 Understanding the Stack and Data Memory

The second paragraph delves into the functionality of the stack, which is a part of the SRAM and operates on a LIFO principle. It is used to store the program counter during function calls, ensuring that the program can return to the correct location after the function is executed. The explanation uses the analogy of stacking plates to clarify the concept of the stack. The paragraph also discusses data memory, which is volatile and stored in SRAM, and its role in setting values during program execution. Special function registers are introduced, which are crucial for the microcontroller's operation, including timer controls, status registers, and port controls.

10:08

🔄 Special Function Registers and Peripheral Interaction

This section explains the role of special function registers in interfacing with peripherals. It describes how these registers act as an interface between the microcontroller's core and external devices, such as LEDs or LCD panels. The paragraph outlines the process of configuring peripherals through software to operate in various modes and the importance of special function control registers in this process. It also touches on the data transfer process involving special function registers and the need for further control data during operation.

15:09

⚙️ Configuration Word and Microcontroller Settings

The final paragraph discusses the importance of the configuration word in determining the microcontroller's operating features. It is highlighted that the configuration word is written during the programming process and cannot be accessed during normal operation. The paragraph explains the various settings that can be adjusted in the configuration word, such as code protection bits to prevent software theft, power-up enable bits, watchdog timer settings, and oscillator configurations. The importance of correctly setting these options during the programming phase is emphasized, and an example of checking for an internal RC oscillator in the datasheet of the 16F-84A microcontroller is provided.

Keywords

💡Microprocessors

Microprocessors are the central processing units of computers and other electronic systems, responsible for executing program instructions. In the video, microprocessors are the focus as the speaker introduces the concept and its significance in embedded systems design, using the book 'Designing Embedded Systems with PIC Microcontrollers'.

💡Program Memory

Program memory, also known as flash memory in the context of microcontrollers, is non-volatile storage that retains the program instructions even when power is removed. The script describes how program memory is organized and its importance for storing the assembler or C code, with specific reference to the memory addresses ranging from 0 to 3FF.

💡Stack

The stack is a special area of memory in a microcontroller that follows a LIFO (Last In, First Out) principle. It is used to store the return addresses for function calls, allowing the program to resume execution at the correct location after the function completes. The script explains the stack's role in function calls and its operation with push and pop actions.

💡Program Counter

The program counter is a register in a microcontroller that holds the memory address of the next instruction to be executed. The script mentions that the program counter starts at address zero upon power-up and is crucial for the sequential execution of code, with specific mention of the interrupt service routine starting at address 004.

💡Data Memory

Data memory, often stored in SRAM (Static Random-Access Memory), is volatile and loses its contents when power is removed. It is used to store data that the program manipulates during execution. The script discusses data memory's role and its distinction from program memory, with examples of general-purpose registers and special function registers.

💡Special Function Registers

Special function registers are dedicated registers in a microcontroller used to control and interface with various hardware features and peripherals. The script explains that these registers, such as TRISA and PORTA, are crucial for controlling input/output operations and are part of the data memory section.

💡Peripherals

Peripherals are external devices or components that connect to a microcontroller to extend its functionality, such as LEDs, LCD panels, or sensors. The script describes how peripherals interface with the microcontroller through special function registers and data transfers, emphasizing the role of these interfaces in connecting the microcontroller to the external world.

💡Configuration Word

The configuration word is a set of bits in a microcontroller's program memory that determines certain operating features and is written to during the programming process. The script highlights the importance of the configuration word in setting up the microcontroller's operation, such as code protection, power-up configuration, and oscillator settings.

💡Code Protection

Code protection is a feature that prevents unauthorized reading of the program memory, protecting intellectual property and ensuring that the developed software cannot be easily copied. The script discusses the role of code protection bits in the configuration word and their importance in safeguarding the software during manufacturing.

💡Oscillator

An oscillator is a component that provides the clock signal necessary for the microcontroller's operation. The script mentions the importance of configuring the oscillator correctly, with options for internal (RC) and external oscillators, and highlights common issues related to oscillator configuration when troubleshooting a microcontroller.

Highlights

Introduction to microprocessors and usage of the textbook 'Designing Embedded Systems with PIC Microcontrollers'.

Overview of program memory and stack, sections 2.3 and 2.4 of the textbook.

Explanation of flash memory as non-volatile memory used for program storage.

Program memory space details with user memory locations from 0 to 3FF, allowing storage of up to 1000 instructions.

Introduction to program counter, stack, and program memory (flash).

Description of how the program counter points to the next instruction in program memory.

Explanation of interrupt service routines starting at address 004.

Details about the stack functioning as last in first out (LIFO) memory.

Explanation of how the stack is used during function calls and returns.

Introduction to data memory and special function registers (SFRs).

Description of SRAM as volatile memory for data storage.

Details about data memory sections, general-purpose registers, and special function registers.

Explanation of how to access banked memory sections using the bank select bit.

Overview of special function registers for controlling peripherals and data transfers.

Introduction to configuration word for setting microcontroller operating features during programming.

Explanation of code protection bits to prevent software theft.

Details about power-up enable bits, watchdog timer, and oscillator configuration.

Exercise suggestion to check the datasheet for the PIC16F84A for internal RC oscillator presence.

Next section to cover topics from 2.5 to 2.7 of the textbook.

Transcripts

hello this is Clint Halstead and this is

introduction to microprocessors I'm

using again the designing embedded

systems with pic microcontrollers

principles and applications second

edition with Tim well first today we're

going to be talking about program memory

and the stack will be going over section

2.3 and 2.4 of the book also will cover

data memory and special function

registers interfacing with peripherals

and talk about the configuration word so

the program memory and stack looks

similar to this diagram the program

memory is made up out of flash flash

memory which is non-volatile memory

which means that you know when the power

is removed from your from the chip then

it remembers the values the program the

program memory is a section of the chip

where you're you're actually your code

exists so your assembler language code

where your C code exists in this section

here called called the flash ok and you

can see here that we have from 0 to 3 FF

of this user memory space program memory

space your instructions are here each

you have each memory address location

even though they're not all shown these

numbers go from 0 0 0 0

hex - 0 0 0 1 0 0 2 0 0

0:04 and it just continues to go on up

even though they're not labeled so you

have a thousand memory locations in

order to store your code notice that

three F F remember F F is represents

eight ones so f f hex it represents

eight binary ones which also represents

256 decimal so 256 and notice that we

have we're going to have four of those

because we have zero one two and three

because it goes all way up to two three

F F that means it had you have 0 F 0 F F

which is 256 and you have one F F 2 F f

+ 3 F F so that's a 1000 you know if you

take 256 times 4 it gives you roughly a

thousand so you have a thousand

different memory locations in order to

store your code so as long as your code

isn't longer than that then it will fit

in this space if you need more code

space than you have to go up to a part

of different pic microcontroller that

has more program memory space or flash

so also notice here that we have we have

several features of the program memory

and the stack first thing we have is

there's three pieces three main pieces

we have the program counter we have the

stack which we'll talk about later and

then we have or we'll talk about in just

a few minutes so we have a program

counter the stack and then we have the

program memory which is the flash now

the program counter points to the

program memory so the the program

counter contains your next instruction

so when powers reset on the chip or when

you first powered up the program counter

is set to zero which means it's pointing

to the very first location which is zero

and then this is where your code has to

start

so also notice you have a also have an

interrupts interrupt service routine

that must start at zero zero four we'll

talk about that later on in the book but

not today

so if you have an external pin that's an

interrupt if that pin goes high

it creates an interrupt and then your

code will instantly go to this place and

so you need to put any sort of interrupt

code here so typically what people do is

they if they're not going to use

interrupts they they reset to zero and

then they put a go-to instruction as the

first instruction so say go to down here

and then you can start your code from

that point so you need to skip this

peripheral interrupt vector so that's

just a little tip we'll learn again more

about that later next thing that we want

to notice here is we've all read about

this a program must start here at zero

and the program counter points to the

locations in the program memory so the

program counter points here now the

question is what is this stack and the

best way I can describe the stack is

it's just a you've got some SRAM here

that will the program counter will be

copied to this stack and the stack is

just some some RAM some memory and this

memory is called a LIFO the book talks

about on page 37 it's a last in first

out last in first out just means that

the last thing that you put in the stack

is the first thing that comes out so if

you for example if you anytime describe

it as being stacking plates you know if

you stack a bunch of plates the last

plate you put down is also the first

plate you're gonna pick up when you grab

it you're not going to grab the plate

from the bottom you're gonna grab it

from the top so that's another reason

they call it a stack and the best way I

can describe the stack is the stack is

used during function calls so when

you're in here using your program and

you're you're running sequentially in

your code if you have a go-to statement

or a call statement I mean

rather a call statement what a call

statement does is it takes the he

remembers the place that you're at the

current value of the program counter

let's say that you I think about it also

as a reading a book you can think of it

as flipping through the pages of a book

and each page of the book you know how

can represent the address to your at so

let's say you're going out through here

and you're executing instructions to

address twenty you can think of that as

being reading through a book and then on

page 20 of the book it says well I want

you to go to page 50 so if you go to

page 50 you need to remember that you're

on page 20 so you need to remember that

you're at address location 20 so if you

jump down here to 50 you execute the

instructions at 50 but then when you get

a return statement you have to remember

where you you have to go back to so you

have to go back to 20 so that's kind of

a way to remember it's the same way when

you're reading a text book when you're

reading along and there's a reference to

another page you have to go to the other

page but you want to also remember where

you are at so that you can continue

reading your text later on so that's

kind of the way it works is is that the

the stack when there's a call function

it it jumps to the location where your

your program is that in the in the your

in your code so it's going to jump to

the new location but it needs to

remember so it pushes that we call that

pushing divided to the stack so let's

say that you're you're executing code to

get to 20 and then you get a call

function to another location it's going

to push that location of 20 onto the

stack now you're going to go execute

your code down here at a different

location and then when you get a return

it's going to what we call pop it's

going to pop that 20 value back to the

program counter so that your you can go

back and continue executing your code so

it's called when you put something on

the stack or when you put the program

counter on the stack it's called pushing

when you pull it back off then it's

called

popping so push and pop okay and go

ahead and read the textbook if you need

more information on that the next thing

we're talking about is data and memory

and special function register map now

the data memory is stored in the SRAM

portion SRAM is volatile memory which

means that these values are lost

you know once the powers are moved from

the chip okay so these do not get

remembered so that basically means your

your code your code typically sets these

values as the as the program is running

so as the program is running here and

your non-volatile memory in your flash

you're going to code that so that it

sets these values the way you want so it

doesn't really matter that these values

go away when power goes away because

your program is going to control these

registers anyway so from that

perspective you know you don't really

need to worry about it so you don't

really need this to be non-volatile

memory but anyway these are called your

special function registers so your your

data memory section is from 0 to 7f but

whether or not this part down here is

grayed out but pins on what type of chip

you have so if you have a larger chip

then you may have memory space all the

way down to 7f H or 7f hex but for the

16 f-84 we only have from 0 to 4 F okay

so means we have 68 general-purpose

registers if we had and notice that it

goes from it goes on both side you have

two banks so if you had a larger chip

that had all of these available than you

would have up to 256 because FF

represents 256 in 7f preferences 128 so

Bank 0 has 128 locations my coin has

another hundred twenty-eight locations

if you add those together it gives you

256 locations but notice that the first

few from zero to zero B are reserved the

rest of these you can write whatever

values you want to these and you can

define them to what you want but the

first 0 to 0 0 to 0 B are called special

function registers this is for the timer

this is for the this is the program

counter low value this is the status

register this is the file select

register for indirect addressing a

support a which controls your your port

a on your chip port B controls port B

which is your pins on your chip your

parallel port then you have EE data

that's your key EEPROM memory location

data and EEPROM address and then you

have some internet interrupt registers

then when you go to banker when you go

to this other side to bank one you

actually the only way you can get there

is you have to set the bank select bit

in the status register to 1 I believe

that's a bit bit 5 of the status

register you have to set two more in

order to go over here and then once you

get that makes a lot select register set

then you go ahead and you you you

address location 0 but that will give

you a 0 so it's a little confusing but

once you see if you have to have this

Bank select register set to 0 to get to

this side and you have to have the bank

select bit set to 1 to get to this side

over here once we do a lab we'll we'll

understand that a little bit better so

the option register you have to be you

have to have the bits so like the make

select bit set to 1 but once you do then

you can get over here option register

notice that some of these are the same

so notice that the program counter low

is the same whether I'm banks you or

Bank 1 same with the status

registers same with the file select

register but notice that for the port a

when you have the bank's select bit one

then you have this tryst a function

instead of port a so you have the tryst

a controls the direction for port a so

if you so what that means is if you want

do you want to report any to B inputs or

outputs so that's what's being set by

the tryst a whether or not the port a is

input or output because you can't really

have both you typically have your ports

or either one way or another it is

possible to have a by Direction report

but you would have to control that

somehow through software through a

read/write bit or something like that

but for a lot of simple app simple

applications they're usually either set

to either inputs or outputs so that's

the tryst a is how you control those and

then your e address EE prom data you

control with these EE control registers

1 & 2 and so that's about it for that

so that's your data memory and special

function register map now when you want

to interface with peripherals you do

that through the special function

control registers which means these

registers like tryst a tryst B and then

the data transfer special function

register which would be port a import B

so those are examples of this kind of

diagram this is a generic diagram of

showing you how do you get to the

outside world

you got this microcontroller core and

you got your program space over here in

your program code and you're wanting to

get to this outside world which may be

an LED or LCD panel liquid crystal

display or a variety of other things but

the way the interface between those the

outside world and the internal processor

core are your special function registers

and your data your data so your control

special function and your data transfers

special function so you've got two

registers here that you need to control

and then you know sometimes your outside

world you have some interrupts that come

into your my

processor so the peripherals can be

configured in software to operate a

number of modes and you do this to do

this certain control data must be

sitting set sit to them to set them up

in the desired way once in use there

will be data flow between core and

peripheral there may still be need for

further control data these needs are

commonly met by means of dedicated

memory memory registers sometimes called

special function registers this approach

gives a microcontroller manufacturer

greater flexibility to extend the

microcontroller family so now we're

going to continue on to the

configuration word configuration word is

important part of the microcontroller as

well it determines certain operating

features of the microcontroller it is in

program memory but cannot be accessed in

normal operation okay so you get this

special word in here it's written to

during the programming process so the

only way to control this configuration

word is when you program the chip when

you connect your picot 3 or pick it to

what are we using to your processor and

program it through your mplab software

there's a special menu at the top where

you can control this but this is not and

many times you can add this some code

some lines of code to your software

program your assembler or C and you can

get these bits to be changed as well

they're called assembler directives so

you can add these are similar directives

at the head of your program in order to

get these to be set but these are for

some pretty important bits here we have

code protect bits code protect bits will

prevent someone from stealing your your

software so if you spend a lot of time

designing some software for a pig

microprocessor or if a company does and

then you spend you know months and

months developing it and then you

competitor just simply takes your chip

and then just reads all your software

and they can just steal your code and

whereas you would put spend a lot of

money developing that then they could

just steal it so the code protection bit

prevents them from being able to read

that back it also prevents you from

being able to read that back but you

don't really need to read that back I

guess it's not really that important it

would be important during debug process

but once you've got the program working

the way you want it and in manufacturing

and you're gonna program all these chips

to go to sell them then you would

probably want to enable the code

protection so zero represents that all

the memory is code protected so if these

are all ones and it would there would be

no code protection so you also have bit

three you have a powerup enable bit you

have a watchdog timer in a little bit

many times the watchdog timer needs to

be disabled when you do many things so

this is a common thing that you would

disable would be the watchdog timer and

then everyone needs to know how to set

the oscillator because the chip won't

work if you don't have an oscillator so

if you have any errors universe first

start trying to use a chip I would check

the watchdog timer and the oscillator

the RC oscillator is the one that's

internal to the oscillator and then you

have these high speed options X TLP

low-power option so these last three

here are external oscillator options and

the RC one is the internal oscillator if

your processor has an internal

oscillator not all of them do so you

have to check your device datasheet to

see if you have an an RC oscillator so a

good exercise here would be to go for

the sixteen f-84 a would be to look at

the datasheet and tell me whether or not

this chip has an RC internal oscillator

if it doesn't you have to add an

external oscillator crystal and wire

that up and configure that that's it for

this section the next section is going

to cover section two point five to two

point seven

thank you very much