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Old 13th Apr 2019, 12:48 am   #52
Slothie
Octode
 
Join Date: Apr 2018
Location: Newbury, Berkshire, UK.
Posts: 1,287
Default Re: MK14 programming interface - MK2

Quote:
Originally Posted by Timbucus View Post
Take any measurement accuracy with a pinch of salt: XIN shows a clock pulse on 0.5uS per div with just under four peaks across two divisions so probably 3.8MHz so a bit low for the 4MHz crystal - it's not really a square wave but a sine wave which I assume is due to the limits of the scope - I was sure it was 20MHz but, maybe I was wrong.
That looks about right for a 4Mhz signal on a 20MHz scope. Don't forget the "20 MHz" is the analogue bandwidth, the point at which the signal level of a sine wave input is attenuated by 3dB or 50%. To make a square wave look square you need a bandwitdth many times the base frequency because it is these higher frequencies that make the corners of the waveform look square.

Your 5v looks really clean compared with what I got with a Switched Mode supply! Mind you I was using a really cheapo Orwon digital scope someone gave me, my best scope (a Phillips/Fluke 4 chan 100 MHz scope is in storage at the moment.)
The SC/MP divides the XIN frequency internally by 4 so activity on the power pin is due to sudden high current usage internally so the pu;ses on the power rail may just be caused by 1Mhz pulses delayed by tiny amounts across parts of the chip, so don't worry if they don't conform to what you expect. Also your scope has input capacitance so applying it to XIN may 'detune' the crystal cscillator - c1 is only 22-33 pF. and many scopes have input capacitance of 50pF. In some cases applying a scope to a quartz crystal can actually stop the oscillation!
You might get a more accurate frequency reading by looking at XOUT - or you might stop the oscillator as mentioned. This shouldnt do any damage though.

Those Hameg scopes are nice scopes BTW, a friend of mine had one and it was really lovely to use for basic scoping around.
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