Is anyone else seeing a huge power fluctuation?

[stephen_usher]

I'm seeing this rather large power and ground spike being generated by the TF536. It's precisely 2us after /IPL2 goes low.

IMG_2078.jpg

(Both traces have 0V at the same level on the 'scope, the blue is +5V and yellow GND.)

I know it's being generated by the TF as if the processor is idle (or if /IPL2 is inactive) there's no spike.

If I switch my processor board over to the 68000 then there is no spike even though /IPL2 is operating in the same way and shows the same signal strength. (Though there is a very small ripple at that point.)

So, is anyone else seeing something on the power rails?

[stephen_usher]

OK. Found the issue. It was my 10K pull-up on the /RESET line. I removed that and the spike disappeared, at least from the power. There's still a spike on the /RESET line.

[stephen_usher]

Nope! Not that, it's back again.

[exxos]

If you connect your scope to GND (both GND and probe) what do you get on the scope then ?

[stephen_usher]

Both on the TF then it's a flat 0V, as you would expect. The spike is relative to the PSU and ST ground.

The resistance between TF GND and ST GND is 0.2 ohms.

P.S. I've cut the +5V trace which goes right across my board and I'm powering that line from a pin right next to the socket. That short (~2 cm) trace is picking up the noise on the ground fill it's passing through when left unpowered with nothing in the socket.

[exxos]

The TF536 has a solid gnd layer, and half the inner layer is 5V the other half is 3.3V. So its got a huge slab of copper there. Spikes from PSU ground to basically anything will likely have spikes. Its why caps are important to give a low impedance down to a supply point. The whole TF board is referenced to system ground so the spikes you see are not really a problem at any point. If the system GND spiked 2 volts, then the whole TF536 ground would do the same, so the noise isn't really there as such. As long as the spikes are not on the PSU side, or TF side, then there isn't anything you can really do about it anyway. The only noise on the TF rails would probably be <100mV or something. Depends on how good the regulator is.

[stephen_usher]

I wouldn't be so worried if the spike wasn't showing up on at least some of the address lines on the ST's own bus.

[exxos]

Is this is a original ST then every signal is bad.