[quote author=Fu-Kwun Hwang link=topic=31.msg6684#msg6684 date=1277338079]
It depends on the maximum current you need to flow through the circuit,
For the same RC value,
with smaller R, bigger C: you will have larger maximum current and more energy stored in the capacitor. YOu might need this for C to act as another power device.
with larger R,smaller C: you will have smaller maximum current and less energy stored in the capacitor.you might need this for using RC as timing device.

[quote]m delaying the continous square wave signal through an RC circuit ...[/quote]

I do not understand why square wave can be delayed with RC circuit? The wave form will be changed. It is not a delayed waveform.

Please describe in detail what is your application (and specification). 20 ns is a very short time. If you just need to delay signal, you should use a coaxial cable with length L=3\cdot10^8 \times 2\cdot10^{-9}=0.6 \,\rm{m}.

Thanks for your answer and I'm sorry for not describing the next question clearly. Basically I've a clock signal which can be programmed to any value from (few KHz to 50MHz). This is the serial clock which I'm deriving from a programmable processor. I gave this clock as an input to the RC circuit and the output I'm connecting to the serial port as its clock. Now, I want to check if i give this way and have t=RC = 20ns will still be able to work properly. Hence, I need to make sure the values of R and C I use for the given clock input of few KHz or MHz. In summary, rather than giving the clock directly, i pass it through RC and then check for the data transfer with the peripheral. The peripheral expects good rise time and fall time  but does not  mention the exact values required for it. so i'm checking with the different RC combination, 10ns, 20ns and check if it really works...

Thanks again for your valuable guidance...

Best Regards,