Odd Problems
Odd problems– are there any other kind?
I did more work on BMOW today, after a few weeks focusing my attention elsewhere. Wow, there are a lot of strange problems going on. Without having changed a thing from my previous boot logo demo, I found that the machine wouldn’t boot up at all. At power-on, I saw nothing on the LCD and garbage on the video. This seemed to have something to do with the USB connection, oddly enough. With no USB connection, the machine would not boot. With the USB connection to a PC, it would boot OK.
After more experimentation, though, the situation appeared less clear-cut. Yes it would boot when the USB was connected, but it would sporadically reset itself every minute or so. And no it wouldn’t immediately boot when the USB was disconnected, but if I left it sitting long enough, it would eventually boot. With no USB, it appeared to be resetting itself every few seconds or so. Sometimes it would crash half way through drawing the boot logo, or draw the boot logo incorrectly in fascinatingly bizarre ways. I tried to capture one of these with the camera, but it always reset itself before I could snap a photo.
My first guess is that I’ve got some kind of power problem. BMOW has a power-on reset chip that will force the reset line to its activate state if the power supply strays too far from 5 volts. Maintaining a USB connection may somehow help minimize power problems, since the USB connector provides power. That may explain why it seems to work better when connected to USB than when not.
After the machine had been on for about 20 minutes, it seemed to work more reliably, although still not without occasional mystery resets. Could the warming of the chips and board have made a difference? Hmmm…
Edit: I should have pulled out the multimeter right away instead of just musing on things. Power and ground differ by 4.5 to 4.6 volts, not the 5.0 volts it should be. The reset chip (a TC1232) is configured to reset the machine if the power is more than 10% out of tolerance, which means outside the 4.5v to 5.5v range. So that certainly explains why BMOW is sporadically resetting. Now as to why the power is out of spec, that may be a more difficult problem to solve.
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I was able to boost the voltage to 4.88 volts just by wiggling the power wires. This thing is precision engineered, I tell you!
Clearly I need a better connection to the power supply. Right now it’s going through a series of three hand-soldered adapters between PSU and board. It’s possible the wire gauge isn’t heavy enough too, although I did some calculating many months back and convinced myself it was fine. I think BMOW draws about 2 amps, but I can’t measure it directly without cutting yet another connection into the power cabling. The PSU is rated for 4 amps, and measures at 5.18v with nothing connected to it.
With power at 4.88v (actually now reading 4.85v as I type this), things seem to be working much better. BMOW has been up for 10 minutes so far with no resets. The LCD contrast also looks markedly better. I’d noticed that the contrast seemed to have been fading over the past month or two, but never paused to wonder why.
With BMOW on, I measured the voltage with and without USB connected, and it didn’t make any obvious difference. But with BMOW off, I discovered that the USB connection somehow maintained BMOW’s supply voltage at 0.41 volts, rather than letting it go all the way to zero. Surprisingly, this seems to be enough to preserve the contents of RAM! You can turn BMOW off for several minutes and leave it off, and as long as USB is connected, programs and data in RAM will still be there when you next power-on the machine.
Hm, that’s pretty neat, you can keep your RAM stored 😀
Yeah, it is a handy side-effect, but all in all it’s probably a bad thing. The USB port shouldn’t be semi-powering the machine when it’s turned off– the USB and BMOW power supplies should be isolated. There’s also some risk of damaging parts when I pull them on/off the board, if “off” isn’t really off.
I began to suspect power troubles when you wrote about the symptoms in an earlier post. It’s essential to have good, low-impedance power and ground wiring in a gadget like this. Try tracing the voltage along the power wiring from supply to system, looking for where the voltage drops. Also make sure you have plenty of decoupling capacitors!
Eeek!
I have it admit, you’re going along famously.
Keep at it, you are indeed working you’re way past your first steps into a new Universe.