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Vampire 600 V1: Take over the Bus
Posted on Friday, May 11 2012 @ 17:32:22 UTC by majsta

Today I worked on this project for just 3 hours because I didn't have much time. Today goal was to investigate how to disable MC68000 from Amiga 600 board and replace it with TG68 core used by Accelerator board. So in this situation we will have Accelerator acting as a Master device. Key signals used in this operation are BR - Bus Request, BG - Bus Granted and BGACK - Bus Grant Acknowledge. After some research I found out that there are two ways to arbitrate the bus on MC68000 processor, 3-wire and 2-wire bus arbitration. So I needed to find out what type is used by Amiga hardware designers. Looking at Amiga 600 schematics I found out that BGACK is pulled high and that was sign to me that 2-wire bus arbitration can be used. Simple understanding of 3-wire bus arbitration is: Accelerator board activates BR, after that MC68K activates BG, Accelerator sends BGACK to MC68K. After everything is done on the Bus BR signals is deactivated, MC68K deactivates BG and Accelerator confirm that deactivating BGACK signal. In 2- wire bus arbitration BGACK is not used. I wanted to simplify this by taking BR and BGACK low because those are their active states and decided to ignore BG signal but that was not best solution su further investigation is needed here.

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