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Crystal tester.

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gojjin

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i have done this project which is crystal tester...range 4Mhz -24Mhz simply it verfies if crystal is actually giving out frequency as it is specified... and i choose to do the AND gate for more accuarcy . As it is opened trough the code frequency will flow in and it is counted for 1sec and it is closed again. the result will be displayed.

my problem is that i hav been questioned.. Why did you use 74HC08 and not 74LS08.. ???? the thing is that i tought it is high speed gate CMOS... can any one help me whats the difference between them and if it is better than using LS since this project is ready assembled and writeup is finished all i have to answer this in the next presentation! and he wants more explanation why i used CMOS
 

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i have done this project which is crystal tester...range 4Mhz -24Mhz simply it verfies if crystal is actually giving out frequency as it is specified... and i choose to do the AND gate for more accuarcy . As it is opened trough the code frequency will flow in and it is counted for 1sec and it is closed again. the result will be displayed.

my problem is that i hav been questioned.. Why did you use 74HC08 and not 74LS08.. ???? the thing is that i tought it is high speed gate CMOS... can any one help me whats the difference between them and if it is better than using LS since this project is ready assembled and writeup is finished all i have to answer this in the next presentation! and he wants more explanation why i used CMOS

I think LS is low power schottky. If the LS part can operate at the speed you need it to and it is lower power than the HC device (may not be) then it would be a better choice. Otherwise, perhaps cost or availability justify your decision?
 
Look at the worst case speed of the 74HC08 when the power supply is 5V. It will not work at 24 MHz. The 74LS08 will.
 
The 74HC08 should work at 24MHz.

If speed is an issue, then go for a higher speed CMOS familly like AC.
 
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