ADCs clock question: r-c combination

[neelam]

ADC's clock?

Hi friends,
A small question!.....i'm using ADC0804 with 89c51. for ADC should i take R-C combination for clock or clock itself from Microcontroller's crystal via flip-flops? or if you can Please explain me when are both the methods used.

Thanks a lot.....

 

[cmos babe]

Re: ADC's clock?

I think that it depends on what you want to do but you should remember that the datasheet recommends fclk=640Khz for good accuracy.Also,the datasheet mentions that the analog signal is converted to an 8 bit digital signal in 64 clock cycles so the sampling rate if you use an RC combination such that fclk=640Khz will be fs = 640khz/64 = 10Khz ==>conversion time is (100 µs)...

I think your µControllers' crystal will provide a higher frequency than the typical fclk =640khz .... and as i've just read from the 89c51's datasheet that it can operate at up to 24 Mhz which is much higher ...... unless there's some way to divide it or something (im no expert)

One technique I've done before was to set the ADC to free running mode (ie tie the INTR\ pin to WR\ pin with CS\ tied to ground ,conversion starts after sending a pulse and conversion will start at high-to-low transition.Your µController then will be operating continously and you won't have to worry about control signals, then you can read the ADC's digital output whenever you want ,but keep in mind that the µC frequency is in Mhz range and the ADC's frequency is 10Khz so the µC may read the ADC's output before the conversion of next sample is done,so you may end up reading same value many times ...It depends on how you design your program..