Radko:~$ cat specTWM-V0.0.1.c
/*
PortA - pin 0 - 3kHz
PortA - pin 1 - 2kHz
PortA - pin 2 - 1kHz
*/
#include <16f84.h>

#define PER_2 1000000/2000 //Period of frequaecy 1000000/Hz=[uS]
#define PER_3 1000000/3000 // -||-

#USE DELAY( CLOCK=4000000 ) /* Using a 4 Mhz clock */
#FUSES RC,NOWDT,NOPROTECT,NOPUT
/* Use RC mode, No Watch Dog, No Code Protect, No Power-up Timer */

#byte port_a=0x05 /* define the location of register port_a */

main()
{
    set_tris_a(0); /* set port_b to be outputs */
    port_а = 0;
    while( TRUE )
    { /* forever loop using WHILE construct */
        //first half period
                port_a = 0x07; //111
                DELAY_US(PER_3);
                port_a = 0x06; //110
                DELAY_US(PER_2-PER_3);
            port_a = 0x04; //100
                DELAY_US(PER_3-(PER_2-PER_3));
                port_a = 0x05; //101
                DELAY_US(PER_3);
        //second half period
                port_a = 0x02; //010
                DELAY_US(PER_3);
                port_a = 0x03; //011
        DELAY_US(PER_2-PER_3);
        port_a = 0x01; //001
        DELAY_US(PER_3-(PER_2 -PER_3));
        port_a = 0x00; //000
        DELAY_US(PER_3);
    }