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The Hitachi 6309E CPU is a direct replacement for the 6809E in the CoCo. What are the options to upgrade to this processor?

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Your first option is to remove the 6809 from the CoCo. Some CoCo 1 and 2 models, all you have to do is remove the CPU from a socket and put a new one in. Other CoCo 1 and 2 models, and all CoCo 3 models have the CPU soldered directly to the PCB. You can use either destructive or non-destructive means to remove the existing CPU; that choice is up to you.

  • Destructive means clipping the legs of the CPU, removing the chip, then desoldering the legs and removing them
  • Non-destructive means using a desoldering tool with a vacuum attachment, or other means, to remove the solder from the legs of the chip and PC board Sometimes this turns destructive, YMMV.

Once the chip is out, you have three options:

  • Option one: Solder the new CPU directly in place in the CoCo

  • Option two: Solder a socket in place in the CoCo

  • Option 3: You can stack a 6309 on top of your computer’s existing 6809. Here’s how…

Cut pin 39 (TSC) of the 6809, leaving a small piece sticking out of the processor so you can solder to it.

Take a 40 pin IC socket. Cut off pins 5, 6, 33, 36, and 38. Bend out pin 39 so you can solder to it.

Stack the 40-pin IC socket on top of the 6809. Solder all of the corresponding pins together. DO NOT solder the pins 39 together.

Now you have two additional options:

1) Solder a wire from 6809 pin 39 to +5V, and another wire from socket pin 39 to ground.

2) Solder a 4.7K resistor between each pin 39 and +5V. Then take a SPDT switch, connect the common contact to ground, and connect each end to a separate pin 39.

In either case, finish up by plugging the 6309 into the socket. If you picked option 1, you now have a 6309 system. If you picked option 2, your system is now switchable between the 6809 and the 6309 (before power-up – not while running!).

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  • Your second solution is clever. Why are the resistors required when they wouldn't otherwise have any (without a switch)? – wizzwizz4 Jul 15 '17 at 15:16
  • @wizzwizz4: It's necessary to drive pin 39 of one chip with +5V while pin 39 of the other is connected to ground. The pin doesn't require much current, so connecting to +5V through a resistor will work just fine. An alternative would be to use a DPDT switch to connect one chip's pin to +5 and the other chip's pin to ground, but using an SPDT switch and resistors is often easier. – supercat Jul 30 '18 at 21:20
  • I have a vague recollection of people doing that piggy-back solution to double the RAM on a ZX80. Basically all the pins were soldered together except for the CS chip select, which was driven by different gates to select a different base address for the chip. I also love the "sometimes this turns destructive" bit - I remember the old "blow-torch the PCB underside followed by a sharp whack on the edge of the bench" method we used to use :-) – user6464 Nov 24 '18 at 0:47
  • But I'd tend to go for the option 2 socket, possibly even a ZIF one if you can find it. You never know when Hitachi may bring out another, better version :-) – user6464 Nov 24 '18 at 0:50
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If your lucky enough to have a socketed Microprocessor from the factory, the process is removing the old chip and installing the new chip.

You do have to make sure your new 6309 is the E version, E for external clock. You also have to make sure your new part can handle the 1.8 mHz clock speed for a CoCo 3, or 0.89 mHz clock speed of the CoCo 1 and 2.

If your CoCo does not already have a socket, you must carefully de-solder the microprocessor and install one.

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  • Must? I would have said "should" but it's still good advice. – user6464 Nov 24 '18 at 0:51
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To make sure you have a chip that will handle the needed clock rate, look for the letter between the 3 or 8 and the 0. A 68A09E or 63A09E can run at up to 1 MHz. A 68B09E or 63B09E can run at up to 2 MHz. There are 63C09Es that can run at up to 3 MHz.

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