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The mini-68k's reset circuit is genius! [message #10020] Mon, 18 July 2022 12:54 Go to next message
sparky62 is currently offline  sparky62
Messages: 2
Registered: June 2022
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The mini-68k's power-on reset that reads the reset vector from rom in the first 8 bus cycles is genius!

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Re: The mini-68k's reset circuit is genius! [message #10021 is a reply to message #10020] Mon, 18 July 2022 14:25 Go to previous messageGo to next message
robg is currently offline  robg
Messages: 43
Registered: October 2017
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While John Coffman IS a most totally excellent gnarly righteous designer dude, this particular circuit dates back to at least 1987 in Wilcox, Alan D., "68000 Microcomputer Systems: Design and Troubleshooting.", page 216.

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Re: The mini-68k's reset circuit is genius! [message #10022 is a reply to message #10021] Mon, 18 July 2022 15:13 Go to previous messageGo to next message
robg is currently offline  robg
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Registered: October 2017
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Well, make that at least 1984 as seen in AN-897, "68008 Minimum System Configuration"

http://bitsavers.org/components/motorola/_appNotes/AN-0897_M C68008_minimum_configuration_system_%5bMotorola_1984_12p%5d. pdf

-- Rob

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Re: The mini-68k's reset circuit is genius! [message #10025 is a reply to message #10022] Tue, 19 July 2022 20:49 Go to previous messageGo to next message
mikesmith is currently offline  mikesmith
Messages: 80
Registered: March 2018
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You don't really need a counter; an R/S flop off the ROM select line and a little combinatorial logic will do the same thing. A mate of mine used to joke that 68000s had 7474 detectors; no 7474, no work.

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Re: The mini-68k's reset circuit is genius! [message #10037 is a reply to message #10025] Sat, 30 July 2022 01:36 Go to previous message
tomstorey is currently offline  tomstorey
Messages: 7
Registered: January 2022
Junior Member
I use a method whereby a certain address line going high clocks a flip flop, and the output of the flip flop feeds into the address decode circuit.

So on reset, the entire ROM space sits from address 0, and after the CPU reads the initial PC and jumps there, the address bit that is part of that value causes the ROM to remap into that space so that code can begin executing there.

There are a few ways to skin this cat. Smile

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