Page 215 - Making PIC Microcontroller Instruments and Controllers

212 CnEAtI GACCURAIE I IEBVALS WllH T|[ER3! THE METhONOIE3

TIMER2

I This is an 8 bit timer
I Official name: TMR2.
I Register used: TMR2.
I This timer can be wdllen to and read fuom.
t Has a register it can be compared to in order to generate an mtemrpt
I The special 8-bit register that is compared to the timer can be written to- An htenupt
is generated when the compadson yields the specified match.
t The timer can be tumed on and off.
I Uses a prescalar,
I Uses a postscalar.
a Cannot use an extemal clock and so carrol be I'lsed as a counter-
I Generates a progranmable intelrxpt.
! Shares cenain functions with the watchdog timer This is important.
I This timer has a special function that is used to drive a synchronous pol1 for trans-
fe ing dala between the PIC and a computer

DEFINIIIG SOME TERMS
Before we start using the timers, let's define a lew timer-related words and concepts that
will help us understand the fr.rnctions of the timers more easily.

HPWM Commands When using the HPWM commands, a timer must be assigned
to help generale the frequencies used by these cofimands. The timer assjgned to this
function is then not available for any other use. In genelal, each timer can be used to
support one function at one time and is busy to all other lunctions. (The deiaull timer
for IIPWM use is Timerl, the 16-bit timer).

Pr€scarar A prescalar is a divider function that slows down the rate at which the inter-
rupts are generated by the timer Il a prescaler is set to 8, the timer clock feed will be
divided by eight times before if is fed to the timer count. This slows down the rate at
which the intefiupts occur by eight. Presca]ars are specilicd by setting a fcw bits in one
of the registers that control the timer. The registers and prescalar values are different
for each timer

Posbcalar A postscalar has lhe same effect as a prescalar! it 1oo slows down the
rate at which the interrupts occur, but instead ofslowing down the clock feed, it counts
the overflows. If a postscalar is set lbr 4, the timer will ovetflow four limes before an
intenupt is generated.
The prescalar anal postscalar values are multiplied by one aoother to get the total delay
seen on the lnlerrupls,

,rterrupt An intenupt is a signal that informs the system that something needs imme-
diate attention. The inteffupt manifests itselfby setting a bit sonewhere in the processor-