Page 44 - Making PIC Microcontroller Instruments and Controllers

IIMEBS 33

Bit TRISE.6 Buffer status 1=Still holds word
o=Has been read

Bit TRISE.T Input buffer status 1=Full
O=Nothing received

Read the datasheet a couple of limes to get a better understanding of these operations.
The precedhg is a very bdef ovefliew and is intended only to alet you and gjve you
an idea of what the possibilities are.

PORTE

PORTE is only 3 extemal bits wide and is a bidjrectional poll. The oiher bits are inter-
nal and are used as mentiooed earlier under PORTD (to which they are related). AII exter-
nai rhree pin' can also be u\ed a\ analog inpul\.
All the pins on PORTE hav€ Schmitt irigger input buffers when used as digital inputs.
The altemate function of the PORTE pins dre denned as follows:
Pin REo direction control of Ph PORTE.0 / RD / AN5

Pin RE1 direclior control of Pin PORTE. I / WR / AN6
Pin RE2 direction control of Pin PORTE.2 / CS / AN7

TIMERS

The PIC 16F877A MCU has four timers: a 'vafchdog timer and thr€e regular timers.
These allow the accurate timing (and counting) of chronological events. Timers are dis-
cussed in much g&ater detail in chapter 6.
Some of the timers have (pre- andpost) scalars associated with t}Iem that can be
usedto multiply the timer setting by an integer amount.As youcan imagine, the scal-
ing ability is not adequate to allow all timed inte als to be created. We also must
consider the unce(ainty in the frequency of the clocking crystal, which is usually
not exactly what it is stated to be (and may drift with ambient temperature) This
means that though fairly accurate timings can be achieved with the hardware as
received, additional software adjustnents may have to be added if more accurate
results are desired. This is doneby having the software make a correction to the time
every so often, (This also means tiat an extemal source, that is at least as accurate
as the result we want, is needed to verify the timing accuracy ofthe device created )
As always, the three limers in fhe microconfoller are clocked $ a fourth of the oscil-
lator speed, meaning that a timer within a 4 MHz system clock gets a counting signal
at I MHz