I am using a FOX F1100e 0614 20 MHz Oscillator , the stk500 and a mega48. Working on the USART, With my scope probe connected to only ground I get a huge 20MHz signal. Scope is a DSO-2100M from www.linkinstruments.com
In a situation like this, where your problem is a (self-inflicted) EMI problem, the best description of your circuit is a couple of photographs.
I assume this is the probe-ground AND the probe tip connected to a ground post on the stk500. This means that in order to get a display like this, you must have a couple volts of ground bounce on the STK500. To get this, the oscillator must be some way away, powered by something else, with discrete GND and signal wires going across the table to the STK500 (ie. not a coax feed).
Another way to get a plot like this is to have one probe connected to the clock signal, and only having the other channel's probe ground clip in use, or use both probe grounds but use 1x mode on the probe connected to clock.
Having a long wire with clock on it hanging off the board half a meter or so will work too.
The only ground I had was through portB 10 pin header, I have now connected the stk500 ground pin on the left side of the board to the breadboard and that reduced the signal by quite a bit.
The power to the board is from PORTB 10 pin header VTG
The second plot is a ripple that shows I am getting 5.12V maximum and 4.8V Minimum connecting probe1 to positive rail. Probe 2 is connected to ground, both ground clips connected to ground as well.
So I am getting .32 mv and .24 mv variation in the voltage.
How can I get these voltages to vary less and is the 20MHz signal I am getting probbly the best I can do with the wiring the way I have it. What is a better way to connect a 20MHZ oscillator or should I use a crystal on the stk500 for testing instead?
Switching to the internal oscillator, i did a plot of the USART output, as you can see I have spikes to 6.16V and -1.36V With the 20MHZ running on the board.
Removing the 20MHZ oscillator I still have the spikes but the 20MHz noise is gone.
Is there something I can do about these spikes or is it the probes and a cheap $750.00 external scope connected via parallel port on the PC?
Try using a proper PCB, those solderless breadboards aren't really suitable for that sort of circuit.
Leon Heller G1HSM
Agree with Leon.
The solderless board and the long "flying" wires give you signals that are totally meaningless with respect to a real circuit board.
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Due the lack of proper PCB if the comms works do not worry about it. The spikes could be overshoot from the scope probes. Have you equalized them?
As for the 20 Mhz consider the length of leads involved and you will realize the scope probe and wiring is picking up all that mush.
Keep it simple it will not bite as hard
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