Author Topic: Newbies guide to CPU, PIA, RAM chips & speeds when fixing MPU boards.  (Read 412 times)

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Offline Marty Machine

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Here's a quick check of the main CPU chip(the brain), PIA & RAM chips used on about 90% of all pinball MPU boards.

It's common practice to try to fix MPU boards by simply swapping/proving if certain chips are dead or not.
However, A VERY common oversight is that people tend to replace these chips with the wrong SPEED replacement, therefore causing the MPU board not to boot, or cause very odd gameplay behaviour *if* it manages to bootup.

The PIA & RAM chips MUST be at the same speed capability (or greater) than the main CPU chip.

The motorola 68xx family of Chips are available in 3 speeds:
  • "Regular" e.g. 6800, runs at 1 MHz.
  • "A type" e.g. 68A00, runs at 1.5 MHz.
  • "B type" e.g. 68B00, runs at 2 MHz.

    Regular   "A" type   "B" type
    1.0 MHz   1.5 MHz   2.0 MHz
CPU:    6800    68A00    68B00
CPU:    6802    68A02    68B02
CPU:    6808    68A08    68B08
CPU:    6809    68A09    68B09
RAM:    6810    68A10    68B10
PIA:    6820    68A20    68B20
PIA:    6821    68A21    68B21

Looking at the 3 coloured columns, it is preferable to make sure that ALL CPU/RAM/PIA chips are from within the same colour group shown above.
This will ensure the speeds of the chips are all matched and will work correctly together.

For example:
  • a RED (1MHz) 6800, should also be using a regular 6810 ram chip, along with regular 6820/6821 pia chips.
  • a BLUE (1.5MHz) 68A00, should also be using a regular 68A10 ram chip, along with regular 68A20/68A21 pia chips.
  • a GREEN (2MHz) 68B00, should also be using a regular 68B10 ram chip, along with regular 68B20/68B21 pia chips.

The *ONLY* exception to the above rule, is that the RAM & PIA chips can be FASTER than the CPU chip.
So, a 1 MHz cpu with 2 MHz RAM/PIA's is quite acceptable.
This is because the CPU will never have any signals that are 'too fast' for the RAM/PIA's to process.

A common problem:
Using a fast CPU (e.g 2MHz) with a slow RAM or PIA (e.g. 1 MHz) will cause all kinds of errors such as switches not being decoded, switches double-triggering, score displays ghosting & jittering numbers etc etc.
This is because the 2 MHz CPU will be overflowing the rate of data sent to the RAM/PIA's, therefore they'll be missing data and creating all kinds of random behavior.

A quick note about CPU speeds.

The Motorola 68xx family of CPUs feature an internal "divide by 4" clock divider.
This means the internal data rate of the CPU will be processing instructions & data at 1-million-times-per-second, aka "1 Megahertz", aka "1 MHz".
Therefore, for a 1 MHz CPU, the external clock/crystal must be 4 times the chip speed ( 4 MHz).

Looking at many MPU board diagrams, you will find many 68xx CPU's use either a 4MHz, 6MHz or 8MHz crystal (or clock generator circuit).

  • a 1.0 MHz CPU needs an external 4 MHz crystal/clock generator.
  • a 1.5 MHz CPU needs an external 6 MHz crystal/clock generator.
  • a 2.0 MHz CPU needs an external 8 MHz crystal/clock generator.

So, if you have an MPU board laying about that has been too hard to fix, it might be a simple matter of matching all your chip speeds ;-)

Marty Machine.
« Last Edit: June 07, 2014, 07:19:50 PM by Marty Machine »

Offline ddstoys

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What a great breakdown Marty

Offline Ballywannabe

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Thanks Marty, useful info ^^^

 I will have to check a couple of my machines that have minor but annoying niggles to see if this could be the problem.

Offline oldskool1969

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I understand a bit more now, thanks.
Might have to get out my sys 11c board from Dr Dude again and have a look at the chip as it had a U25 ram failure and just locked up. So I bought a new rotten dog board and had issues with that too!
On my Space Station I took out heaps of 6821 PIA,s along with transistors and micro drivers in a catastrophic fail, still don't really know how I did it though? Lucky I guess  %.%
If it isn't broke, it isn't pinball.

Offline Limorange

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Thanks for sharing the info Marty, it brings a better understanding on the ins and outs.
Definitely put that on my check list for fault finding.

Offline mitchjjam

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Interesting about the clock speeds. I guess it is like PCs you can have faster ram than the CPU bus but not the other way or it will slow down the CPU.

Offline ralph67

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Great information Marty , really helpful
 Now where is my printer , Ill keep a copy handy  ^^^