Page 81 - Making PIC Microcontroller Instruments and Controllers

READIIIG THE II{PUIS 7.I

program with the scanning of the nibbles in PORIB added to dete.mine what happened
and when it happened.
A loop scans the high nibbte ofPORTB, the outpur from the keyboard. When all
4 bits are pulled high, rhis nibble will be reaal as HEX F. If it is HEX F, no keys are down
and we rescan the keys. If, howevet a key has been pressed, the answer will be orher
than HEX F and can be interpreted as follows:

If 84 is low the answer wi]I be HEX E (15 t=14) 10 Column I
If85 is low the answer will be HEX D (15-2=t3) 01 Cotumn 2

It86 is low the answer will be HEX B (15 4=11) 10 Colunn 3
IfBT is low the answer will be HEX T (15-8=7) 0tLtCotumn4

To determine which ro\'r' the key ihat was pressed is in, we have to know which of
the bits in rhe low nibble had been faken LOW by the scanning rouline.
The values ior the low nibble are as follow..

IfB0 is low the low nibble will be HEX E (15-l=14) I I l0 Row I
IfBl is low the low nibble witl be HEX D (15-2=t 3) 1tol Row 2

ff 82 is low the low nibble will be HEX B ( 15 4=ll) I0 Row 3
IfB3 is low the low nibble willbe HEX 7 (15-8=7) 011I Row 4

Haying Ihe tuo pieces of preceding infomation lets us identify rhe key thar was
pressed. No matter how many keys we have and no maner how they are laid out, the
scanning routine to read the keyboard will be something like what wasjust explained.
Next, in Program 5.14 we will display the contents ofthe entire byte on the first line of
the l,CD so we can actually see what is happening in the regisrer reFesenteal by PORTB
as we scan the lines. Then, on the second line we will show the low byte and the high byte
sepantely so we can see what each key press does. We have added a l/20 second delay in
the loop (so we can se€ the scanned value), so we have to holal each key down for at least
l/20 second for rhe scan to make sure the key press will register and show in the display.

*!8in*.!!i61tr, Read keyboard (Reading lhe keyboard rows and cotumns)
Ct.EAR
DEFINE OSC {
DEF1NE ICD_DREG PORTD LCD defines follou
DEFIIIE IiCD DBIT 4
DEFTNE LCD RSREG PORTE
DEAINE LCD RSBIT O
DEFINE LCD EREG PORTE
DEFINE IJCD EB1T 1
ADCONI - 800000111 nake PORTA and PORTE digital

(conti uea