14 Ağustos 2007 Salı

Assembly Language Programming CHAPTER 3

Introduction

3.1 Representing numbers in assembler
3.2 Assembly language elements
3.3 Writing a sample program
3.4 Control directives

define
include
constant
variable
set
equ
org
end
if
else
endif
while
endw
ifdef
ifndef
cblock
endc
db
de
dt
CONFIG
Processor
3.5 Files created as a result of program translation



Introduction

The ability to communicate is of great importance in any field. However, it is only possible if both communication partners know the same language, i.e follow the same rules during communication. Using these principles as a starting point, we can also define communication that occurs between microcontrollers and man . Language that microcontroller and man use to communicate is called "assembly language". The title itself has no deeper meaning, and is analogue to names of other languages , ex. English or French. More precisely, "assembly language" is just a passing solution. Programs written in assembly language must be translated into a "language of zeros and ones" in order for a microcontroller to understand it. "Assembly language" and "assembler" are two different notions. The first represents a set of rules used in writing a program for a microcontroller, and the other is a program on the personal computer which translates assembly language into a language of zeros and ones. A program that is translated into "zeros" and "ones" is also called "machine language"

Physically, "Program" represents a file on the computer disc (or in the memory if it is read in a microcontroller), and is written according to the rules of assembler or some other language for microcontroller programming. Man can understand assembler language as it consists of alphabet signs and words. When writing a program, certain rules must be followed in order to reach a desired effect. A Translator interprets each instruction written in assembly language as a series of zeros and ones which have a meaning for the internal logic of the microcontroller.
Lets take for instance the instruction "RETURN" that a microcontroller uses to return from a sub-program.
When the assembler translates it, we get a 14-bit series of zeros and ones which the microcontroller knows how to interpret.

Example: RETURN 00 0000 0000 1000

Similar to the above instance, each assembler instruction is interpreted as corresponding to a series of zeros and ones.
The place where this translation of assembly language is found, is called an "execution" file. We will often meet the name "HEX" file. This name comes from a hexadecimal representation of that file, as well as from the suffix "hex" in the title, ex. "test.hex". Once it is generated, the execution file is read in a microcontroller through a programmer.

An Assembly Language program is written in a program for text processing (editor) and is capable of producing an ASCII file on the computer disc or in specialized surroundings such as MPLAB,which will be explained in the next chapter.

3.1 Representing numbers in assembler

In assembly language MPLAB, numbers can be represented in decimal, hexadecimal or binary form. We will illustrate this with a number 240:

.240 decimal
0xF0 hexadecimal
b'11110000' binary

Decimal numbers start with a dot, hexadecimal with 0x, and binary start with b with the number itself under quotes '.


3.2 Assembly language elements


Basic elements of assembly language are:

Labels
Instructions
Operands
Directives
Comments
Labels

A Label is a textual designation (generally an easy-to-read word) for a line in a program, or section of a program where the micro can jump to - or even the beginning of set of lines of a program. It can also be used to execute program branching (such as Goto .......) and the program can even have a condition that must be met for the Goto instruction to be executed. It is important for a label to start with a letter of the alphabet or with an underline "_". The length of the label can be up to 32 characters. It is also important that a label starts in the first clumn.


Instructions

Instructions are already defined by the use of a specific microcontroller, so it only remains for us to follow the instructions for their use in assembly language. The way we write an instruction is also called instruction "syntax". In the following example, we can recognize a mistake in writing because instructions movlp and gotto do not exist for the PIC16F84 microcontroller.

Operands

Operands are the instruction elements for the instruction is being executed. They are usually registers or variables or constants.


Comments

Comment is a series of words that a programmer writes to make the program more clear and legible. It is placed after an instruction, and must start with a semicolon ";".

Directives

A directive is similar to an instruction, but unlike an instruction it is independent on the microcontroller model, and represents a characteristic of the assembly language itself. Directives are usually given purposeful meanings via variables or registers. For example, LEVEL can be a designation for a variable in RAM memory at address 0Dh. In this way, the variable at that address can be accessed via LEVEL designation. This is far easier for a programmer to understand than for him to try to remember address 0Dh contains information about LEVEL.



3.3 Writing a sample program

The following example illustrates a simple program written in assembly language respecting the basic rules.

When writing a program, beside mandatory rules, there are also some rules that are not written down but need to be followed. One of them is to write the name of the program at the beginning, what the program does, its version, date when it was written, type of microcontroller it was written for, and the programmer's name.

Since this data isn't important for the assembly translator, it is written as comments. It should be noted that a comment always begins with a semicolon and it can be placed in a new row or it can follow an instruction.
After the opening comment has been written, the directive must be included. This is shown in the example above.

In order to function properly, we must define several microcontroller parameters such as: - type of oscillator,
- whether watchdog timer is turned on, and
- whether internal reset circuit is enabled.
All this is defined by the following directive:

_CONFIG _CP_OFF&_WDT_OFF&PWRTE_ON&XT_OSC

When all the needed elements have been defined, we can start writing a program.
First, it is necessary to determine an address from which the microcontroller starts, following a power supply start-up. This is (org 0x00).
The address from which the program starts if an interrupt occurs is (org 0x04).
Since this is a simple program, it will be enough to direct the microcontroller to the beginning of a program with a "goto Main" instruction.

The instructions found in the Main select memory bank1 (BANK1) in order to access TRISB register, so that port B can be declared as an output (movlw 0x00, movwf TRISB).

The next step is to select memory bank 0 and place status of logic one on port B (movlw 0xFF, movwf PORTB), and thus the main program is finished.
We need to make another loop where the micro will be held so it doesn't "wander" if an error occurs. For that purpose, one infinite loop is made where the micro is retained while power is connected. The necessary "end" at the end of each program informs the assembly translator that no more instructions are in the program.

3.4 Control directives

3.1 #DEFINE Exchanges one part of text for another

Syntax:
#define []

Description:
Each time appears in the program , it will be exchanged for .

Example:

#define turned_on 1
#define turned_off 0

Similar directives: #UNDEFINE, IFDEF,IFNDEF


3.2 INCLUDE Include an additional file in a program

Syntax:
#include
#include "file_name"

Description:
An application of this directive has the effect as though the entire file was copied to a place where the "include" directive was found. If the file name is in the square brackets, we are dealing with a system file, and if it is inside quotation marks, we are dealing with a user file. The directive "include" contributes to a better layout of the main program.

Example:
#include
#include "subprog.asm"


3.3 CONSTANT Gives a constant numeric value to the textual designation

Syntax:
Constant =

Description:
Each time that appears in program, it will be replaced with .

Example:
Constant MAXIMUM=100
Constant Length=30

Similar directives: SET, VARIABLE

3.4 VARIABLE Gives a variable numeric value to textual designation

Syntax:
Variable=

Description:
By using this directive, textual designation changes with particular value.
It differs from CONSTANT directive in that after applying the directive, the value of textual designation can be changed.

Example:
variable level=20
variable time=13

Similar directives: SET, CONSTANT


3.5 SET Defining assembler variable

Syntax:
set

Description:
To the variable is added expression . SET directive is similar to EQU, but with SET directive name of the variable can be redefined following a definition.

Example:
level set 0
length set 12
level set 45

Similar directives: EQU, VARIABLE

3.6 EQU Defining assembler constant

Syntax:
equ

Description:
To the name of a constant is added value

Example:
five equ 5
six equ 6
seven equ 7

Similar instructions: SET

3.7 ORG Defines an address from which the program is stored in microcontroller memory

Syntax:
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Cascode stage
or “collector follower”
Jean-Paul Brodier
All microprocessors from the 8051 family have inputs and outputs that are ‘quasi-bidirectional’. This means that when power is first applied, the ports behave as inputs with a logic high level and a weak pull-up.,

Glitch
a relay or some other load such as When driving an optocou-pler or LED, there is a problem
at power on: the NPN transistor in the common emitter connec­tion (Figure 1) causes an unde­sirable excitation of the load from the moment power is applied until the microprocessor has had the chance to turn the output low. In addition, logic high outputs are seldom able to deliver enough current to drive the transistor into saturation because they have been designed to be active low.
Figure 1. An NPN transistor drives a load.
To solve both of these problems in one hit, we have to make the active level logic low. This can be done in three different ways: use an emitter follower as a buffer stage (Figure 2a), an inverter in a common emit­ter circuit (Figure 2b) or an inverter/open collector circuit (Figure 2c). The disadvantage of solution 2a is the fact that the voltage to the load is reduced. In the case of a relay with a 5-V coil there is the risk that the resulting voltage is too low. The disadvantage of examples 2b and 2c is that they require more parts.
Collector follower
That leaves the open collector buffer in the form of an IC type 7404. This solution, however, also has a few disadvantages. You do not always need all of the 6 buffers in one IC. Also, the SMD version can only handle 12 V. This is too low and dan­gerous if we happen to supply the load from an unregulated voltage.
The solution presented here com­bines in one transistor the advan­tages of the emitter follower (inactive when power is first applied) and open collector (higher power supply voltage, lower current). This circuit has been known since the valve era by the name cascode (drive via the cathode). The goal was to reduce the Miller-effect of the internal (parasitic) capacitances. Not having the option of reduc­ing the capacitance between the internal electrodes, a lower volt­age was used instead. The cas-code circuit is often used in pow­erful transmitters (tens of kW) to minimise the Miller-effect. This circuit was also used to limit tran­sistor conduction and to keep the dissipation within bounds, which increased the life of bipolar tran­sistors. This was in the IGBT and VMOS era.
The transistor conducts only when the output from the micro­processor is low (refer Fig­ure 3). The base current is lim­ited by resistor R. This current is determined by the current flow­ing through the load. When the power is switched on, both the base and emitter see the same potential, VCC, so the transistor remains blocked. One thing we have to keep in mind: we may not exceed the current rating of the microprocessor output because it has to cope with all the current flowing in the emitter of the transistor.
In the case of the quite common 80C51, this maximum current is typically 3.2 mA (two LS TTL loads). This is sufficient to drive an LED without overloading the 5-V regulator, or for driving a PNP power stage at the high side (Figure 3b). The parallel Philips PCF88574 I2C interfaces can handle 25 mA. For the Atmel AT89Cx051 as well as for the Philips P89LPC9xx the limit is 20 mA. For the latter type the cascode circuit or ‘collector fol­lower’ is even more interesting when the outputs are configured as open-drain because the nom­inal voltage is only 3.6 V. In all cases we have to make sure that the maximum dissipation of the
Figure 3. Cascode driver stage with discrete transistor.
package is not exceeded. 24 V is sufficient to energise its are determined by the power
Should this be the case, then the half Watt relay coil, which in PNP (or VMOS) transistor.
number of open collectors turn can drive a load of 16 A at The cascode transistor can be a required will probably justify 230V. ‘digital’ type with integrated resorting to a 7404. For loads driven from the positive base and emitter resistors.A current of around 20 mA at side, the voltage and current lim-


pot as interrupt generator

In battery-powered, microcon­troller driven circuits, as well as with microcontrollers operating in cars, it is desirable to switch the micro into power-down mode once a task has been completed. An interrupt request is then required to wake up the micro. This circuit allows an interrupt to be generated in a simple way using a common potentiometer. In the example circuit, the pot may also copy its spindle position to the ADC. This enables the pot to be used for continuously variable settings (like volume) as well for getting the micro out of its power-down mode.
IC1A is configured as a differen­tiator with R3 preventing oscillation by keeping the gain down to 10 times. Because the opamp oper­ates off a single-rail supply voltage, an 18k/10k potential divider (R1/R2) is able to create a virtual ground level at +1.75 V. This can be done because the LM358 can handle input levels of up to 3.5 V when supplied at 5.0 volts. IC1A supplies a brief High pulse at a falling input voltage, and a similar Low pulse when the input voltage rises. In order to get a High pulse when the potentiome­ter spindle is turned cw or ccw, IC1B is set up as an inverter. Next, each opamp output drives the base of a BC547 transistor. The 5 V-to-0 V transitions at both collector outputs are shaped and combined into a usable interrupt pulse by three NOR gates IC2A, IC2B and IC2C.If the potentiometer spindle is turned very slowly, it is possible that the circuit does not respond
That is why an LED has been added that lights briefly when a pulse is generated. Finally, a tip: a 100-pF capacitor may be connected in parallel with R5 for additional suppres­sion of self-oscillation.

elektor time standart


Elektor Time standard (1988)
Jan Buiting

The Elektor Time Standard and associated Slave Unit were spin-offs of another hugely successful project, the DCF77 Receiver / Locked Frequency Standard. The receiver was published in the January 1988 issue, the Time Standard and Slave display in the next two issues. All units were housed in then very fash­ionable and (expensive!) Ver-obox two-part ABS enclosures which had also been used for a number of Elektor test instrument designs published between 1984 and 1987. The Time Standard box was designed to process seconds pulses received from the VLF (77.5 kHz) DCF77 time standard transmitter in Mainflingen, Ger­many, and display time (with atomic accuracy) and date on an LC display. The circuit was based on then extremely popular 8052AH-BASIC microcontroller from Intel, a device, we can safely claim, that made it to fame & glory thanks to Elektor Electron-
ics. The 40-way DIL chip con­tained a BASIC interpreter capa­ble of executing ‘tokenised’ code from an external EPROM. This, we were told by our resident designer Peter Theunissen, made writing the DCF77 time signal decoding routines ‘a doddle’ using his specially adapted BASIC computer and interpreter. For example, when concerns were raised (by myself) that not all of Europe was in the time zone served by DCF77 (i.e., CET or GMT+1h), a menu option was quickly added to allow users to select between UTC and GMT+1h. As a relative novelty, a ready-made self-adhesive front panel foil with built-in membrane keys was designed into the proj­ect. This expensive item had been produced specially for Elektor. However, when the article went into print (using a rather glum page layout and black & white print), there were yet other con­cerns regarding the range of the DCF77 transmitter. This is offi­cially claimed as “approximately 1,000 km by groundwave propa-

gation”. A quick use of a com­pass and a map of Europe sug­gested that the signal would only cover the south-eastern part of the UK, possibly including Greater London. For a couple of months we waited with baited breath for readers’ responses, only to receive two enthusiastic reception reports, one from the East coast of Ireland and another from Riyadh, Saudi Arabia! The latter report came from a reader work­ing at a chemical laboratory. I remember he wrote that DCF77 could be received for a few min­utes a day only, synchronising the clock, usually around nightfall despite heavy ‘static’. A huge wire antenna was used (nothing like the 1-inch ferrite rods we used in our lab, which is less than 100 km away from Mainflingen). Although the BASIC program list-
ing for the Time Standard was freely distributed to interested readers (on paper, in an enve­lope, by snail mail!), only very advanced readers were able to compile the program into tokenised code and burn it into an EPROM. Most other readers had to rely on a ready-pro­grammed 27C64 supplied through our Readers Services. Apart from displaying time and date at atomic accuracy, the Time Standard was also capable of outputting time/date information

in the form of ASCII character strings for other (intelligent) equip­ment to use, for example, a timer or switching clock. Although sales figures of the PCB and EPROM were in the hundreds, I never heard from anyone actually hav­ing enjoyed the wonders of the ASCII output so extensively described in the article. The Slave unit published in March 1988 was connected to the Time Standard via screened (micro­phone) cable, the idea being that one or more Slave units could be installed on walls in rooms at some distance from the main clock unit. Central timekeeping deluxe for offices, labs, schools and workshops, but at what an expense and design effort! Not too many PCBs were sold for this extension of the Time Standard.