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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: desantii on October 06, 2016, 03:38:54 AM

Title: A4000 int/ext jumper
Post by: desantii on October 06, 2016, 03:38:54 AM

Hi, looking for clarity as to what the jumper actually does. Let's say in an a4000 I am using a 3640 running at 30mhz vs the stock 25. With the jumper in EXT what is it actually doing? telling the mb to use the clock from the CPU card (in effect running the mb at  30mhz) OR is it telling the CPU card don't use the mb clock and run with your own crystal?

Thanks

Title: Re: A4000 int/ext jumper
Post by: PPC on October 06, 2016, 07:49:50 AM

When you set the jumper to EXT it will use the crystal on the CPU card.
030 CPU cards from C= did not have an on-board crystal for example so in that case you have to set the jumper to internal.
A CSPPC card has it's own crystal and then you have to set the the jumper to external or else the amiga will not even boot at all (just a black screen)

Title: Re: A4000 int/ext jumper
Post by: psxphill on October 06, 2016, 08:46:04 AM

Quote from: desantii;814922

Let's say in an a4000 I am using a 3640 running at 30mhz vs the stock 25. With the jumper in EXT what is it actually doing?

With the jumper set to INT the motherboard generates a clock for the CPU, with it set to EXT the motherboard doesn't generate a clock.

In either case the motherboard generates it's own clock.

When you use an external CPU clock then the motherboard has to synchronise the two clocks when the CPU accesses the motherboard.