HCF4013 Pain Test mixed results

[JonPyro]

Hi All,

So I was sitting at my bench today working on a latch circuit from the HCF4013 (http://www.st.com/content/ccc/resource/technical/document/datasheet/b7/dc/2d/40/2b/31/4c/b5/CD00000362.pdf/files/CD00000362.pdf/jcr:content/translations/en.CD00000362.pdf) and as per usual i started with a simple RC circuit on the reset so i knew when it powers on it resets Q low.....obviously i worked on some values that made the pulse decay quickly to avoid it lingering between the VIH and VIL thresholds.

But then I thought to myself, why not try and make it hold between the threshold values and see how the chip holds up to any oscillations on the outputs which i assumed would happen as the reset is not being held either high or low.

Below is a trace, yellow on the Reset input and then blue on Q output:



As you can see, horribly slow charge lasting around 50s but nothing much of interest until we reach around 6V and you can see a blip on the output, zooming in we see this:



6-5V is in the lower end of the no go area on a 12V supply so could this be the oscillation I am trying to achieve? This is scaled on the 20mV range so this is tiny tiny tiny where I would have expected it to be a couple of volts at least.

For me this is surprising as I was always taught running a long time in this range can cause certain death but in this case I hardly call a 100mv or so causing many problems. Or maybe this is not what I am looking at at all?

I have triggered this a few times just to make sure its not some weird noise but it consistently appears in the same place and same amplitude.

The datasheet doesn't show this circuit having any real protection against idiots like myself doing this so I was expecting it to be a much bigger more obvious problem....the input shows a buffer but not a schmitt trigger which was my first thought to why it is so suppressed.