WEBENCH® Tools/LMK04208: nonsense results

Christian Rausch66

Prodigy 120 points

Part Number: LMK04208

Tool/software: WEBENCH® Design Tools

Hi,

I tried to find a solution for the following clock tree with Webbench:

input: 43.333333 MHz

output 0: 25 MHz

output 1: 25 MHz

output 2: 5 MHz

output 3: 5 MHz

One of the solutions was a design with the LMK04208.

This solution is my preferred one because I need a digital delay for all the outputs, and the other solutions use different parts without digital delays (which took a little time to find out...).

But:

After pressing "Open design" I get a lot of strange results:

1. Webbench suggests a 43.3333 VCXO. This is useless, because such VCXOs are not available

2. Webbench suggests 1191.6667 MHz as VCO frequency. This is outside of the frequency range of the VCO in the LMK04208

3. Dividing 1191.6667 MHz by an integer never leads to 5 MHz or 25 MHz

Summary:

Webbench seems to be useless for designing clock trees.

Is there another tool that works a little better?

Regards

Christian

over 8 years ago

0 Timothy T over 8 years ago

TI__Mastermind 28550 points

Hello Christian,

Thanks for your feedback. I just did a design with LMK04208 using 43.333333 input reference but got a valid solution as the VCO = 2750 MHz, except yes - my issue was the CLKout0 = 10.8333 and CLKout1 = 2.1667 in spite of the block diagram showing 25 and 5 MHz. outputs.

The issue appears to be the handling of the 43.333333 MHz. This gets converted to 43 + 333333/1000000 and appears to be resulting in a solution in the weeds. What you really want is 43 + 1/3. Clock Architect does allow you to enter frequencies as mixed fractions. This will give you a solution having a 100 MHz VCXO and 2750 MHz VCO.... and the clock outputs actually at 5 and 25 MHz. You can see the design at the following link:
webench.ti.com/.../OpenPublicSharedProject.jsp

73,
Timothy

Clock & timing

Clock & timing forum

WEBENCH® Tools/LMK04208: nonsense results