Usually a frequency counter contains some functional blocks :
The input signal it is applied to a gate G and then to a counter, for a precisely period of time After this period of time , the input signal it is stopped for a while and then the cycle it is repeated. In the time of break the content of counter it is transferred in buffer memory and then it is displayed. After the load of buffer memory , the contains of counter is reset of reason to can begin a new count cycle. The successions of this signals are conform the follows diagram :
The signals COUNT , LE , RESET are generated from the base of time . A very simple base of time which contains 2 ICs can be build using the folow schematic:
Unfortunately this schematics have a long time of cycle Tc = 2s. In order to make this time shorter we can modified only the times Tle and Treset . But that means to use more CIs or another solution is to use a microcontroller with the necessary software. I have used a PIC microcontroller type PIC16F84A because it is easy to programm and the power consumption is low. The program is written for the PIC16F84A but cheaper PICs (as PIC16C61) can be used.
;======= Base of time =================================18/06/00== ; rb0 1s ; rb1,rb2 .75ms ; internal clock ; standard crystal 4 MHz XT ; http://www.module.ro ;------------------------------------------------------------ ; configure programmer list p=16f84A;f=inhx8m _CP_OFF equ H'3FFF' ;code protect off _PWRTE_ON equ H'3FFF' ;Power on timer on _WDT_OFF equ H'3FFB' ;watch dog timer off _XT_OSC equ H'3FFD' ;crystal oscillator __CONFIG _CP_OFF & _PWRTE_ON & _WDT_OFF & _XT_OSC ;------------------------------------------------------------ ; cpu init portb equ 06 count1 equ 0C count2 equ 0D count3 equ 0E ;------------------------------------------------------------ ; bit init w equ 0 f equ 1 ;------------------------------------------------------------ org 0 ; ;------------------------------------------------------------ init movlw 0 tris portb ; set portb as output movwf portb start bsf portb,0 ;rb0=1 call pause ;wait 1s bcf portb,0 ;rb0=0 bsf portb,1 ;rb1=1 call pauza ;wait .75 ms bcf portb,1 ;rb1=0 bsf portb,2 ;rb2=1 call pauza ;wait .75 ms bcf portb,2 ;rb2=0 goto start pause movlw .8 ; load count3 with decimal 8 movwf count3 d3 movlw .250 ; load count1 with decimal 250 movwf count1 d1 movlw .165 ; load count2 with decimal 165 movwf count2 nop ; compensation 1us d2 decfsz count2,f ; decrement and skip next line if zero goto d2 ; if not zero decfsz count1 ; decrement count1 if count2 is zero goto d1 ; do inside loop again if count1 nz decfsz count3 ; decrement count3 if count1 is zero goto d3 ; do inside loop again if count3 nz retlw 00 pauza movlw .250 movwf count1 lop1 decfsz count1,f goto lop1 retlw 00 ;------------------------------------------------------------ end ;============================================================
Download project from here. For the counter I have used two specialized IC's type MMC22926, that works up to 6 Mhz, the National Semiconductor alternative to MMC22926 is 74C926. If a higher frequency is need to be processed, a ECL or ACH divider can be used at gate input.
Base of time schematic using a PIC16F84 microcontroller.
Frequency counter circuit using two specializated IC's type MMC22926.
Display circuit
Back to my home page