AQA Style Microcontroller Simulation - Electronics Tutorials 

Microcontroller with Harvard Architecture - Based on the AQA Spec'

• 8 bit unidirectional address bus
• 8 bit bidirectional data bus
• 6 bit unidirectional instruction bus
• R/W is a unidirectional control bus line determining read or write operations
• Clock is a unidirectional control bus line carrying the clock signal used to synchronise all the microcontroller operations

Each instruction consists of a six bit op code and an eight bit operand.
The op code is fetched on the instruction bus and simultaneously, the operand is fetched on the data bus.

When the power is switched on, or after a reset, code execution starts from address 0x00.

This image shows the buses linking the internal microcontroller devices

The Registers

• W is the working register or accumulator, through which all calculations are performed.
• PC is the program counter or instruction pointer. This value (via the address bus) determines which instruction is fetched.
• SP is the stack pointer used to save the return address when calling a subroutine.
  The stack begins at R[0xF7] and grows towards zero.
   
• SR is the status register containing status flags. These include ...
•     Z, the zero flag which is set if there is a zero result after a calculation or move
•     C, the carry flag which is set if a calculation gives a result too big to store in 8 bits (an overflow)
•     T, the timer interrupt flag which is set when the TMR register is set to zero.
   
• TMR is the timer register. This counts from any assigned number down to zero. When zero is reached, the T flag is set. TMR is decremented after a certain number of clock pulses determined by the PRE register. TMR is memory mapped at address R[0xFF].
   
• PRE is the prescaler. TMR is decremented when the number of counted clock pulses matches the value in PRE. This has the effect of dividing the clock frequency by (PRE + 1). If PRE is set to 0, TMR is disabled. If PRE is set to 1, the clock frequency is divided by 2. If PRE is 255, the clock frequency is divided by 256. PRE is memory mapped at address R[0xFE].
   
• Configuring Input and Output Pins
  TRISA, TRISB, TRISC, PORTA, PORTB and PORTC are used for data input/output. The "tris" data direction registers control tristate logic used to determine whether the port pins are inputs or outputs. If a "tris" register bit is set to 1 then the equivalent port bit is an input. If the "tris" bit is set to 0 then the equivalent port bit is an output. The PORT and TRIS registers are memory mapped at addresses between R[0xF8]and R[0xFD].
  1 = I = Input
  0 = O = Output.
  All port pins default to Outputs because TRISA, B and C are initialised to Zero.
   
• R - there are 256 general purpose registers / storage locations addressed from 0 to 255 or from 0x00 to 0xFF.
  The registers including and above 0xF8 are memory mapped to the I/O registers and the timer and prescaler.

• R[0xF8] is TRISA
• R[0xF9] is PORTA
• R[0xFA] is TRISB
• R[0xFB] is PORTB
• R[0xFC] is TRISC
• R[0xFD] is PORTC
• R[0xFE] is PRE - Prescaler
• R[0xFF] is TMR - Timer

The simulator clock can be set between 0Hz and 120Hz. The upper limit is determined by the maximum Flash player frame rate. With older hardware, the higher clock rates might not be achieved. These slow clock speeds make visible strange effects that are never seen with real-life microcontrollers. For example, seven segment displays switching from 09 to 10 might show 19 for a brief moment because it is impossible to change both digits at the same instant.

The Instruction Set

The code is NOT case sensitive so movw is the same as MOVW and 0XFF is the same as 0xff.
Most features in the AQA specification are now available in the simulator.
Some design assumptions have been made so the exam microcontroller might differ from this one.
K is used to represent a literal, which can be a memory location (e.g. 0x29), a label (e.g. display:) or a value, (e.g. 0xFA).
KKKK KKKK represents the same as an eight bit binary number.
Legal literal names for K begin with _ or a text character.
Literal names must not begin with a digit and must only contain _ : a-z A-Z 0-9.
Literal names must not duplicate reserved words such as MOVW or CALL.
Literal names must be unique or the assembler will not be able to resolve the ambiguity.

R represents a register or memory location.
RR RRRR RRRR RRRR is a fourteen bit binary representation of the same.

XX XX represents a fourteen bit or four digit hexadecimal number.
x (lower case) is a "don't care" value that is ignored.