Page 189 -

176 Chapter Eleven

word is detected, the output (pin 2) drops to ground momentarily. The micro­
controller monitors this line to determine when a word has been detected.
Once a word has been detected, it is necessary for the interface to read the
BCD output from the speech recognition circuit. By using the high­ and low­
digit BCD nibbles, it’s possible to distinguish trained target words. To do so,
the interface must distinguish the error codes 55, 66, and 77 from trained
words numbered 5, 6, and 7. To accomplish this, the interface circuit uses four
NAND gates off the 4011 integrated circuit. The NAND gates are connected to
the high­digit nibble. If the high­digit BCD nibble has the equivalent word
numbers of 5, 6, or 7, the output from the four NAND gates is low. The output
from the four NAND gates is connected to pin 11 (RB5) of the 16F84. The
16F84 reads this pin to determine if the high­digit nibble is a 5, 6, or 7 (0 V or
ground). If these numbers are not displayed, the output of the NAND gates is
high (�5 V).
So far our circuit can tell when a word has been detected and if the result­
ing word is an error code. If the output of the speech recognition circuit is an
error code, nothing else happens; the microcontroller loops back to the begin­
ning of the program, waiting for another word detection. On the other hand, if
a word is detected and it is not an error code, the microcontroller passes the
low­digit number through to the 74HC154 (4­ to 16­line decoder) IC. The
74HCT154 decoder reads the binary number passed to it and brings the cor­
responding pin equivalent to that number low.

PIC 16F84 microcontroller program
The PIC 16F84 used in both interface circuits contains the following PicBasic
program:

‘Speech recognition interface program
symbol porta = 5
symbol trisa = 133
symbol portb = 6
symbol trisb = 134
poke trisa, 255
poke trisb, 240
start:
peek portb, b0
if bit4 = 0 then trigger ‘Trigger enabled, read speech recognition
circuit
goto start ‘Repeat
trigger:
pause 500 ‘Wait .5 second
peek portb, b0 ‘Read bcd number
if bit5 = 1 then send ‘Output number
goto start ‘Repeat
send:
peek porta, b0 ‘Read port a
if bit4 = 1 then eleven ‘Is the number 11
poke portb, b0 ‘Output number