Power And Reset Circuits

A project log for MC68000 Computer

BenchoffBenchoff 12/20/2013 at 23:220 Comments

Since we're using an ATX power supply for this computer, there is a little magic that needs to happen before it becomes a proper power supply. Besides the usual orange, red, yellow and black wires attached to a 20-pin connector, there are also a few extra wires that make the whole thing work. The most important is the green wire, or /PS_ON line. Only when this wire is shorted to ground does the supply turn on. That's a fairly easy circuit to whip up.

Also on the ATX connector is a gray wire. This is the PWR_OK line. This signal is generated by the power supply and is at +5VDC when the power is okay. This is a great place to put a power LED, and is another very easy circuit to put together.

Careful readers will note these circuits were already implemented on the backplane itself. However, since we're going for a slick, streamlined look for this computer, it only makes sense to put add this circuit to the front panel.

The Reset Circuit

Every computer, from the awesome 4-bit builds we see to the i5 warming my ankles as I type this, needs a reset circuit. For the 68000 I'm using, there are actually two types of reset I need to consider: the power-on reset, and the manual reset. Power-on reset is needed when - duh - the computer is powered on. Manual reset is for when I screw something up terribly and need to figure out what went wrong.

Conceptually, the reset circuit for this computer is as simple as having one line on the backplane connected to a switch. When I want to reset the computer, all I need to do is ground the reset line. A simple tact switch takes care of this. It's not that simple, though: without a debounced switch, crazy things happen, the CPU isn’t happy, and looking at what the 68000 is doing with a logic probe is a sure path towards insanity. [Garth Wilson], god of modern 6502 single board computers, has a great site up for circuits used in homebrew computers. In that, he covers a few reset circuits that can be easily generalized to any homebrew project.

[Garth]'s circuit, while perfectly reasonable and easily implemented, doesn't account for a power-on reset. This is somewhat common in the homebrew computer scene, requiring the user to press the reset button whenever the computer is powered on. A lot has changed in the 30 or 40 years since the glory days of homebrew computers, and now there's an awesome one-chip solution for this problem. It's Microchip's MCP130, a small SOT-23 device that will both keep the reset line low until the power supply has reached a stable voltage, and keep the reset line held low for 350ms after the power supply is stable.

Putting These Circuits Together

I designed my backplane with the express intention of putting a power/reset circuit on the front panel. For the sake of modularity, I broke out all 64 pins on the backplane to a 2x32 0.1" header. With this, I can design a front panel that shows the status of the data and address lines (blinkenlights, anyone?), and could potentially create a front panel that's at least as cool as the king of minicomputers. For now, though, I'll simply put the power and reset circuits on the board and drill a few holes in my enclosure.

Below you can see the first iteration of what I like to call the 'frontplane'. I should have checked the +5 Volt line on the backplane was connected to the power plane of this board. Simple enough fix, though: a soldering iron, xacto knife, and hot glue will do wonders for any circuit.

Yes. I put BMO on the silkscreen of this board. Also Football.

As the Hackaday corporate overlords probably don't like trademark infringement, I've removed BMO from the board in this project's Github. I also fixed the issue with the power pin, so if you're building this for yourself, feel free to grab the gerbers and have fun.

That's about it for the power and reset circuits. In the next post we'll take a look at the clock generation circuit and cobbling together something with wire-wrap sockets.