Ultrasonic [HC-SR04] + PIC16F877A

[LinkMTech]

You will also need to change your oscillator to a higher frequency for two reasons:
1. PAUSEUS min time is 24us at 4MHz
2. PULSIN resolution is 10us at 4MHz

I would suggest changing your crystal or resonator to 20MHz so PAUSEUS 10 will work and PULSIN resolution will be 2us.
Then try your modified code to see if it works:

PORTA = %00000000
TRISA = %00000010       ' Bit 1 set as input
PORTB = %00000000
TRISB = %00000000

ADCON0.0 = 0            ' A/D module disabled

define osc 20           
INCLUDE "modedefs.bas"

w var word
r var word
a var word
tr var PORTA.1 'Trig pin
ec var PORTA.2 ' Echo pin

w = 0
low tr

Main :
    high tr
    pauseus 10          ' Minimum 3us at 20MHz
    low tr
    pulsin ec,1,w
    r = w/29            ' Divide by 29, PULSIN resolution is 2us. 1 count every 2us        
    if r < 10 then
    PORTB = %11111111   ' Turn ON all PORTB outputs
    pause 200
    PORTB = %00000000   ' Turn OFF all PORTB outputs
    endif
goto main
end