Low voltage on old CPU, the MC68000 and MC68008 CPUs

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#1
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Hi,

 

I am experimenting with the old CPUs.  Got some from eBay.  They are the MC68000 and the MC68008, both operating at 5V.

 

I basically wire them up on a breadboard by simply hooking up the Vcc/GND pins with 0.1uF decoupling capacitors.  I source the +5V from the the USB port of my laptop.

 

I see that the USB power source gives from +5.0V to +5.11V.  BUT when I measure the voltage the CPU's Vcc/GND pins, I get 3V or so.  Why the voltage drop?

 

Is there something about these old CPUs.  Of course, I could be getting dead CPUs, as buying such things from eBay is not 100% certain.  The same for the Intel 8086 and the MC6809 CPUs.

 

BUT, when I try the modern, if you will, w65c816s CPU, WDC (Western Design Center's) 6502 compatible CPU that is still made today, it shows a strong 5V.  Same for modern micro-controllers like the ATMEGA328, etc.

 

Does this have to do with TTL that is with old CPUs????

 

Thoughts?

 

Thanks!

Last Edited: Sat. May 12, 2018 - 05:10 AM
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Measure the current drain... perhaps it exceeds your USB rating.

 

Ross McKenzie ValuSoft Melbourne Australia

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The older stuff was nmos. These don’t drive rail to rail like cmos does. What does the datasheet say?
I just read the datasheet on the mc68000 - voh (voltage out high) for most of the pins is 2.4V for a 400uA load. Fine for ttl interface.

Last Edited: Sat. May 12, 2018 - 08:16 AM
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He's measuring Vcc/Vdd not a GPIO pin.

 

Buy something like one of these:

 

https://www.adafruit.com/product/1852

 

My one is only 500mA range but is still usefull from time to time when diagnosing USB power problems. It does current * time accumulations also

Image result for USB current meter

No link available anymore. It's evaporated since I bought it.

 

Last Edited: Sat. May 12, 2018 - 09:13 AM
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Not sure if this is the problem but:

 

Back in about 1996 I should use some Atmel 8051 chips (cant remember the number but had 8K flash), but the problem was that we had a ASIC that was the "master" so among other things controlled reset and the clk pin(s), and it did not change the clk before release of reset, that is fine on a intel chip, but the Atmel osc did not like it ! (it used 38mA as I remember), and that that was more that we could deliver in that setup at boot time, (the chip should never use more than about 5mA).

 

So perhaps you have a problem like that. 

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What clock are you providing?  IIRC those nmos chips need an external clock supplied!  Not to mention ROM and RAM too.

 

 

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Last Edited: Sat. May 12, 2018 - 03:05 PM
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ki0bk wrote:

What clock are you providing?  IIRC those nmos chips need an external clock supplied!  Not to mention ROM and RAM too.

 

None.  Per OP, I only connect the Vcc/GND pins, that is, giving it some juice.  All I would like to see if the CPU is actually getting 5V -- very, very basic testing first.  Are you saying that I need to provide the clock input for the CPU to get to the 5V potential?

 

So, I hooked a 555 to provide an astable/square signal of 32ms period, meaning 31.Hz.  Still not working.  Maybe clock freq is too low?  Browsing through the datasheet now to see what the minimum freq is for the MC68000 and MC680008.

Last Edited: Sat. May 12, 2018 - 03:47 PM
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valusoft wrote:

Measure the current drain... perhaps it exceeds your USB rating.

 

 

Yeah, my laptop would scream at me if I pass 900mA, which is USB 3.0.  I even got one of those USB 1A adapter that plugs into the wall to try it out, the CPU still gets only 3.4V or so not fully 5V.

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unebonnevie wrote:
Still not working. 

What do you mean by that?

 

In what way do you expect the chips to "work" without any clock or code or memory ... ??

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valusoft wrote:
Measure the current drain... perhaps it exceeds your USB rating.

Indeed!

 

These chips were quite power-hungry - they would not be expected to run from USB ?

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unebonnevie wrote:
None.  Per OP, I only connect the Vcc/GND pins, that is, giving it some juice.  All I would like to see if the CPU is actually getting 5V -- very, very basic testing first.  Are you saying that I need to provide the clock input for the CPU to get to the 5V potential?

What you’re attempting is pointless. You need to carefully research the datasheets for these antiques since they will require multiple power connections, possibly bi-phase clock sources and external ram and rom to get them to do anything that resembles running.

Most of the processors of that era had dedicated support chips that everyone used to reduce design time. For example, the 8080 had the 8200 series of parts with a clock generator chip, bus interface chip, parallel i/o chip, serial i/o chip and on and on it went. The 68000 had its share of peripheral chips as well.

You cannot just connect 5 volts and learn anything whatsoever. More can be gleaned from reading the datasheets than actually fiddling with a few bits of the total hardware required to do anything. Why you’d want to, other than for posterity, is beyond me. What’s the point of learning how to build or write code on a Lisa or Kaypro? Obsolete and long ago forgotten, unless of course you actually worked with the stuff. Many of us have.

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Etron55 wrote:
What you’re attempting is pointless. You need to carefully research the datasheets for these antiques since they will require multiple power connections, possibly bi-phase clock sources and external ram and rom to get them to do anything that resembles running.

 

Why would one need to hook up ROM/RAM even a clock at Mhz to see if the CPU receive 5V at the Vcc/GND pins?  Even a free-running circuit, which would be my next step, does not need RAM/ROM and can execute NOOP instructions.

 

Etron55 wrote:
Why you’d want to, other than for posterity, is beyond me. What’s the point of learning how to build or write code on a Lisa or Kaypro? Obsolete and long ago forgotten, unless of course you actually worked with the stuff. Many of us have.

 

Some of us chase women and do retro-computing wink

Last Edited: Sat. May 12, 2018 - 04:26 PM
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awneil wrote:

unebonnevie wrote:
Still not working. 

What do you mean by that?

 

In what way do you expect the chips to "work" without any clock or code or memory ... ??

 

read the OP please!

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unebonnevie wrote:
Why would one need to hook up ROM/RAM even a clock at Mhz to see if the CPU receive 5V at the Vcc/GND pins?  Even a free-running circuit, which would be my next step, does not need RAM/ROM and can execute NOOP instructions.

 

Uh, yeah, right, whatever. 

 

I'll reiterate. Read the datasheet. Perhaps you'll learn that the majority of the early processor designs had minimum clock frequencies for a reason. Perhaps not.

 

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unebonnevie wrote:
read the OP please!

See #10.

 

It seems quite likely that what you are seeing is the expected behaviour - so the chip is working.

 

You still haven't answered the question from #1: how much current is it drawing?

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Last Edited: Sat. May 12, 2018 - 07:30 PM
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I think you're having us on here. I looked over your older posts and found this almost identical issue:

https://www.avrfreaks.net/forum/mysterious-voltage-drop

 

Did you not learn anything from that thread ? Ad hoc connections and crappy power supplies just won't do for MC68000.

 

BTW:

Vintage *MOS can be very vulnerable to static discharge. ESD protection on input pins wasn't common in those days. You may have already blown up your processor.

 

I don't remember MC68000 having build-in reset circuits. If you don't reset it properly according to data sheet it will be in an indeterminate state.

 

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N.Winterbottom wrote:
... BTW:

Vintage *MOS can be very vulnerable to static discharge. ESD protection on input pins wasn't common in those days. You may have already blown up your processor....

I can not think of any digital CMOS/NMOS IC from the mid-1970s onward that did not have some level of ESD protection.

 

From the Motorola M68000 data sheet:

This device contains protective circuitry against damage due to high
static voltages or electrical fields; however, it is advised that
normal precautions be taken to avoid application of any voltages
higher than maximum-rated voltages to this high-impedance circuit. 

 

 

- John

Last Edited: Sat. May 12, 2018 - 08:56 PM
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Would not the Motoroala/Freescale/NXP/whomevernow forum be best for this?

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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N.Winterbottom wrote:

I think you're having us on here. I looked over your older posts and found this almost identical issue:

https://www.avrfreaks.net/forum/mysterious-voltage-drop

 

Did you not learn anything from that thread ? Ad hoc connections and crappy power supplies just won't do for MC68000.

 

 

Have you read reply #4 from that thread?  And, no, I don't have you on here or whatever reason you think of.  Why so unfriendly when doing post?

 

No, not crappy power supply!  5V USB 3.0 up to 900mA rating.  Can't imagine the CPU would take more than 900mA, as the laptop would let you know, and the CPU is cool at room temperature.

 

This is variation, although much, much simpler, just one chip and wires to Vcc/GND pins.

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I'm not up with the inner workings of USB3, but the old USB used to default to 100mA and the device would negotiate a higher current. So it could very well be that you're only getting 100mA and thus the voltage drop. So start with a known good supply that will supply at least an Amp. As for just applying power to the chip to see what happens probably won't be too conclusive.

 

I had a look at my first 68k board the other day - it hasn't fared too well as there's a bit of corrosion on the pcb and on the chip cover (ceramic pkg) as well. I've also got a handful of original Mac boards with their full size plastic pkg 68k

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I decided to scale down a bit and use the Z80, instead of MC68000.  I get 4.5V between as Vcc input via a USB power source.  I wired up a free-running circuit and the CPU executes NOP instruction continuously, and I can see, via an address  pin (A2) of the CPU, the led blinks more or less at the frequency produced by the 555 as the clock source.  Looks good.

Attachment(s): 

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Again, how much current is being drawn?

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If you are going to do this sort of work you need to get the following:

 

  • Bench PSU
  • DMM
  • Simple logic analyser

 

then you'll be able to see what is going on.  Simple search gave below:

 

 

Power draw shouldn't be a problem but can't find original datasheet to confirm.

 

David

 

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DAFlippers wrote:
... Power draw shouldn't be a problem but can't find original datasheet to confirm....
Here is the one (2.3 MB PDF), I quoted from earlier. Page 10-8 quotes a maximum power dissipation of 0.13 (8mhz) to 0.38W (20mhz).

- John

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jfiresto wrote:
I can not think of any digital CMOS/NMOS IC from the mid-1970s onward that did not have some level of ESD protection.

Oh I can - In 1978 (or thereabouts) I got through several digital clock IC with integrated 7-segment LED drivers. I can't remember the number. It used this new fangled NMOS technology and despite discharging my hand on a nearby cenrtral heating raditor I managed to destroy these chips by touching the keybutton inputs.

 

My first experiments with 4000 series CMOS also killed several parts. These ICs were just short lived versus the instant death of the clock IC above because as we know now, ESD damage is cumulative.

 

Last Edited: Mon. May 14, 2018 - 07:25 AM
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Some digital ICs may not have had good static protection by today's standards, but I don't remember any that had none. 4000 CMOS was protected up to 1kV, IIRC, which is fairly easy to inadvertently generate. With that always in mind, I followed the chip makers' handling recommendations and only lost parts, in circuit, to lightning strikes.

 

I sometimes wonder if static protection has gotten too good, after one fair-sized distributor sent some unprotected JFETs in a plain, clear PE bag. After I paid them a call and had them look at the datasheet, a picker returned a few minutes later with replacements, packed in individual, pink, dissipative bags! We stopped using them for less than robust parts.

- John

Last Edited: Sun. May 13, 2018 - 11:38 AM