Page 140 - Programming Microcontrollers in C

What are Microcontrollers? 125

are numerous excellent, fast microcontrollers constructed with the Von
Neumann architecture. The Von Neumann architecture has been the
mainstay of microcontrollers and will be the only microcontroller
configuration available for the foreseeable future.
A microcontroller has its program stored internally, and the ALU
reads an instruction from memory. This instruction is decoded by the
ALU and executed. At the completion of the execution of the
instruction, the next instruction is fetched from memory and it is
executed. This procedure is repeated until the end of the program is
found, or the program gets into a loop where it is instructed to branch
back to a beginning point. In this case, the machine will stay in the
loop forever or until something happens to release it from the
never-ending loop.
There are three ways for a machine locked in a loop to be removed
from the loop so it can execute code outside of the loop. These
operations are called exceptions. The first is to reset the part with a
reset signal. A reset signal usually requires connecting the reset pin
of the part to a logic low signal. A logic low is usually ground. When
this condition is detected, several internal registers are set to
predetermined values, and the microcontroller fetches the address of
the reset routine from a specific memory location. This address is
placed in the program counter, and the program starts to execute.
There is a table in memory that contains the addresses of several
routines accessed when exceptions occur. These are the addresses of
the interrupt service routines, reset routines, etc. This table is called
the vector table, and the addresses are called vectors.
A second means of forcing the part out of the loop is for the part
to detect an external interrupt. An external interrupt occurs when the
interrupt request (IRQ) pin on the part is set low. This pin is tested at
the beginning of the execution of each instruction. Therefore, if an
instruction is being executed when an IRQ is asserted, the instruction
will complete before the IRQ signal is processed. Processing for the
IRQ consists of first determining if IRQs are enabled. If they are, the
status of the machine is saved. All interrupts are disabled by setting
the interrupt mask bit in the status register of the microcontroller.
Then the address stored in the IRQ vector location is fetched. This
address, the address of the interrupt service routine (ISR), is placed
in the program counter. The ISR then executes.