ChuckT wrote:
There's a very interesting circuit trick in there. As far as I can tell, he's relying on the 6502 Data Bus not being driven during PHI1. So he's able to connect the address bus to the data bus with a couple of 74244's, and sample the data bus, in order to emulate any peripherals - there are no peripherals otherwise accessible to the 6502. (For a peripheral read, I suppose he drives the databus during PHI2, as normal.)
There's a hint of this and a
click-to-enlarge circuit diagram on
his blog.
Quote:
The reason I was able to do it with so few chips is a combination of the Propeller microcontroller's versatility, and a neat timing trick that allowed me to multiplex the data bus. After reading how old 6502 computers interleaved video access with processor accesses on the same bus, I realized I could use the same technique to allow the Propeller to access the bus in the interim period between each 6502 bus cycle. Since the 6502 puts the address out before the data is read or written, the Propeller can quickly transfer the 16 bit address onto the 8 bit data bus in two transfers, and still finish in time to return the data bus to a "normal" state so that the 6502 never knows anything happened.
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