Measuring clock frequency of a 68000 processor - Page 1
Forum section: electronics / beginners

author: Sanxion
author_rank: Contributor
timestamp: November 13, 2023, 03:16:31 pm
content: Hi all,I am attempting to measure the clock frequency of my Amiga's 68000 processor. Pin 15 is for the clock and should register 7.14 Mhz. However, my oscilloscope fluctuates between 70 and 112 Hz.I should add that my Amiga is working and it must therefore be something I am doing wrong. As you have probably gathered, I am relatively new to using an oscilloscope.The probe is set to 10X.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 13, 2023, 03:39:04 pm
content: Make sure the horizontal scale of the scope is between 50ns to 200ns/div

author: Sanxion
author_rank: Contributor
timestamp: November 13, 2023, 04:09:56 pm
content: Thanks.I have altered the horizontal scale to DIV:100nS - the scope now just shows a flat line. Is there something else that needs to be done?

author: KrudyZ
author_rank: Frequent Contributor
timestamp: November 13, 2023, 04:25:12 pm
content: What scope do you have and how do you probe it (ground?)It would be helpful if you can show a picture...

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: November 13, 2023, 04:32:33 pm
content: You can often pick up oscillator signals using a few turns of insulated wire wound in a coil of perhaps 1/2” diameter. No risk of your probe tip shorting something out.SJ

author: Andy Chee
author_rank: Super Contributor
timestamp: November 13, 2023, 04:50:12 pm
content: The clock signal will probably be 5V TTL/CMOS, so set your vertical division to 1V/div.Also set your input coupling to DC coupling. AC coupling would work as well, but for a TTL clock I prefer DC coupling.

author: Sanxion
author_rank: Contributor
timestamp: November 14, 2023, 08:38:27 am
content: OK, I have changed both the horizontal and vertical settings as advised above. The result is the same, please see the image.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 14, 2023, 09:29:38 am
content: I missed it (and others as well apparently), but for a TTL clock signal your probe should probably be set to 1x, rather than 10x.

author: Sanxion
author_rank: Contributor
timestamp: November 14, 2023, 01:35:45 pm
content: I tried that and now I do have something that resembles a wave and for a very brief moment the Freq registers 7.xx MHz and then reverts back to 0 Hz...then after a few seconds shows 7.xx MHz again. I am almost but not quite there!

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: November 14, 2023, 02:03:19 pm
content: Hi Sanxion,....guess you have to fiddle with the Trigger-level.Good luck

author: CaptDon
author_rank: Super Contributor
timestamp: November 14, 2023, 02:17:15 pm
content: Most scopes make piss poor frequency counters with a lot of 'rough estimate' displayed. Even with a very stable trigger and stable waveform you will see endless least significant digit bounce. You need to measure the CPU frequency?? What crystal is attached? That or a sub-multiple IS your frequency unless something is A.F.U.'d

author: m k
author_rank: Super Contributor
timestamp: November 14, 2023, 05:20:49 pm
content: What next and previous pins are showing?

author: srb1954
author_rank: Super Contributor
timestamp: November 14, 2023, 06:24:15 pm
content: Quote from: Andy Chee on November 14, 2023, 09:29:38 amNo. For most passive probes in the x1 mode the available bandwidth is severely restricted, typically to less than 10MHz, which wouldn't be adequate for viewing the details of a 7.14MHz clock signalMonitoring a TTL signal with the probe set to x10 is perfectly OK with any normal scope. It is preferred to use the probe in x10 mode for most applications as it possesses a higher bandwidth and presents a lesser capacitive load to the circuit under test. The x1 mode is normally only used when viewing very small signals where the scope sensitivity isn't sufficient to allow use of the x10 mode.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 14, 2023, 06:36:18 pm
content: Quote from: srb1954 on November 14, 2023, 06:24:15 pmIn which case, if the OP wishes to use their probe in 10x mode, they should ignore my 1V /div vertical setting, instead it should be 100mV/div.

author: pqass
author_rank: Super Contributor
timestamp: November 14, 2023, 06:49:53 pm
content: If you've set the probe switch to 10x then you also have to tell the 'scope too. Expensive probe/'scopes have extra pins near the BNC to do this automatically.Notice the "[1:1]DC" top, center right of screen; should be "[10:1]" or "[1:10]". Press the channel 1 button and you'll likely find it in a submenu.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: November 17, 2023, 03:52:27 am
content: Do you have a ground connection?The scope probe needs to be hooked up in two places, the signal itself and a ground.Show us how you make your connections!

author: Sanxion
author_rank: Contributor
timestamp: November 17, 2023, 03:04:07 pm
content: The ground connection is made to a "ground" point on the motherboard. I have tried to follow the way I have seen this done on another Amiga as demonstrated by Adrian (Adrian's Digital Basement).

author: wraper
author_rank: Unknown
timestamp: November 17, 2023, 03:23:17 pm
content: Quote from: Andy Chee on November 14, 2023, 09:29:38 amHeck no, at 1x probe will be totally useless at this frequency. And TTL is nowhere near to being that low when you may need to switch to 1X. BTW square signal output for probe adjustment on oscilloscopes has even lower voltage than TTL.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 18, 2023, 03:59:48 pm
content: Sound like an open circuit probe. Test your probe with a function generator.As suggested, a sniffer coil will be a better way of measuring the freq. A probe can pull the oscillator out of tune.I recall from the early 80's (not bad) that the 68000 doesn't have an internal clock osc. Isn't it an external osc module?

author: edavid
author_rank: Super Contributor
timestamp: November 18, 2023, 05:00:20 pm
content: From your scope photo, the signal just doesn't have a high enough on-screen amplitude for the scope to measure it. (A better scope would be able to measure it anyway.)Reduce the vertical scale factor so the signal fills the screen, and it should work fine.(The whole 1X/10X/pickup coil discussion is a red herring, since most 1X probes will certainly work well enough at 7MHz, and the 68K clock is strong enough to drive a scope probe.)

author: tridac
author_rank: Regular Contributor
timestamp: November 18, 2023, 05:44:44 pm
content: Set the scope probe to 10x, volts per division to 0.1, make sure the input is not set for 50 ohm termination, blank trace to 1 division up, trigger level to auto, dc coupling, and time div to 0.5uS / cm. If the scope has autorange, you could try that as well...

author: srb1954
author_rank: Super Contributor
timestamp: November 18, 2023, 08:00:23 pm
content: Quote from: edavid on November 18, 2023, 05:00:20 pmNo it's not a red herring.With a x1 probe just about the only parameter you could measure with any degree of reliability is the frequency of the 7MHz clock.With a x1 probe the viewed 7MHz square wave clock signal would mostly look like a distorted sine wave and other critically important parameters for a microprocessor clock, such as waveform logic levels, pulse widths, rise and fall times, overshoot and undershoot, would not be measurable at all.

author: edavid
author_rank: Super Contributor
timestamp: November 18, 2023, 09:14:29 pm
content: Quote from: srb1954 on November 18, 2023, 08:00:23 pmWhich is the only parameter OP wanted to measure

author: radiolistener
author_rank: Super Contributor
timestamp: November 18, 2023, 10:18:12 pm
content: 1x probe has low impedance and high capacitance, it can easily affect circuit behavior, so it is not recommended to use it.1:10 is good enough for most the cases, except very sensitive circuits which works at very high frequency, such circuit may needs to use 1:100 in order to reduce effect of probe capacitance.It's hard to see the picture, because it has too high resolution and doesn't fit into a screen, but at a glance it looks that he using 5V/div scale which is too high for such signal, needs to try 1V/div.I suggest to check that probe is set to 1:10 mode and oscilloscope setting for probe also set to 1:10. If you use 1:10 probe with 1:1 setting you will get 10 times smaller voltage results...

author: edavid
author_rank: Super Contributor
timestamp: November 18, 2023, 10:29:25 pm
content: Quote from: radiolistener on November 18, 2023, 10:18:12 pm"Open Image in New Tab" works.He has channel 1 set to 1V/division, so it appears he is using a 10X probe, so that is not the issue.