Lpc2000

Joerg

Thanks for the suggestions.

Using a very short ground probe I've got an improved waveform 2.5V pk-
pk but asymmetrical, where the fall time is approx 3ns and the rise 
time is about 8ns.  

Removing the load improves the risetime and I get ~3v pk-pk. My scope 
has a 200MHz b/w & I've looked at a 70MHz clock on another board & I'm 
seeing a nice clean squarewave so I'm happy that the scope isn't 
giving a false picture. It does seem that the XCLK pin is having a 
hard time driving the load. 

I've doubled checked the datasheet & the load input capacitance is 
10pF max. The clock trace is on the top layer & is ~15mm long, 12mil 
wide with no vias. There is a groundplane on the next layer.

Do you have any other suggestions ?

Regards
Dave

--- In lpc2000@yahoogroups.com, "Joerg Schulze-Clewing" <joergsch@...> 
wrote:
>
> Hello Dave,
> 
> > 
> > I'm using a LPC2214 & have connected the XCLK pin to a single 
input of 
> > another IC which needs to be sync'd with the CPU. I'm driving XCLK 
at 
> > 59MHz and I'm seeing an almost sinusoidal waveform ~2v pk-pk going 
> > between 0.5v & 2.5v which is very marginal. Is the XCLK pin 
capable of 
> > driving a 10pF input located 15mm away from the XCLK pin ?
> > 
> 
> 10pF represents around 270ohms at that frequency. I am not familiar
> with the LPC2214 but that doesn't seem like much of a load for a uC.
> Are you sure it's only 10pF?
> 
> Lift the pin of the other IC and look again. If still sinusoidal the
> trace capacitance might be a bit high. If your scope probe is 
suspect
> you could use a resistive divider into a 50ohm coax and hook that to 
a
> fast scope (at least 1GHz) with the scope input terminated with
> 50ohms. The divider should have a ratio of 5-10, not more to avoid
> stray capacitances that could otherwise foul up things.
> 
> If your trace is longer than an inch or two you may need a 
controlled
> impedance trace and a terminator at the target IC.
> 
> Regards, Joerg
> 
> http://www.analogconsultants.com/
>