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Old 20 February 2020, 21:16   #1
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E clock signal in Zorro slot purpose?


I'm exploring Zorro II\III bus wiring and can't figure out for what reason E clock signal exists there. Maybe someone can help me to figure out?
The only guess I have is about _INT2 and _INT6 signals, which are normally generated by CIA's and should be synchronous to E clock (as CIA's are clocked with E clock).
So, if you want to generate your own irq's, as a HW developer you would want them to be similar to those generated by CIA's, and clocking your logic with E clk seems to be a natural way to do it.

But I didn't find any proofs in documentation, so I may be wrong.

Thank you in advance!
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Old 20 February 2020, 21:26   #2
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The E clock is used to communicate with peripherals designed to work with the older Motorola M6800 processor.

Have a look at Appendix B, M6800 Peripheral Interface, in the 68000 User's Manual (page 185 in the PDF): https://www.nxp.com/docs/en/referenc.../MC68000UM.pdf
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Old 20 February 2020, 21:30   #3
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Interrupts don't need to be synchronised with any clocks - you can assert them whenever you like. The original Zorro specification was basically an extension of the 68000 bus, which includes the E clock for synchronising I/O devices. It's kept in the later versions of the specification for backwards compatibility, but as you suspect, it's not really necessary as more advanced peripherals would use their own clocks and proper bus arbitration instead. Besides, it's 1/10th of the master clock frequency of ~7MHz, so is too slow for governing full speed bus access.
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Old 20 February 2020, 22:03   #4
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Thank you Niklas, Daedalus!

I found some interesting info here: http://amigadev.elowar.com/read/ADCD.../node0292.html

So, connecting 6800 peripherals was never legal, and actually is impossible with later machines. But E clock remained in Zorro III for compatibility even when it was removed as an output from cps's along with 6800 bus support. Weird.

Last edited by confusion; 20 February 2020 at 22:23.
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