[Coco] MC6809 Inards

jdaggett at gate.net jdaggett at gate.net
Tue Oct 3 16:09:18 EDT 2006


My what you can find out when you start reading patents. I was searching the US patent 
records for anything that might be related to the MC6809 and actually found about four or 
five patents that could be directly related to the processor. There is no one single patent that 
is, in whole, a complete description of the processor. Instead there are several patents when 
combined, give insight to the internal workings of the microprocessor. I even found several 
patents regarding the MC6820/21 PIA chip. 

Some interesting things that I have found:

1) The MC6809 and the MC6801/3 are probably more closer related internally than the 
MC6809 is to the MC6800. An interesting thing is the main CPU processor core schematic 
for the MC6801 is included in patent # 4266270, Microprocessor having dual internal data 
busses. Beware this is a huge schematic if you want to print it out. It prints out to about 28 
letter size pages. 

2) It appears that there are three internal clocks in the MC6809/01/03 and maybe even the 
MC6811/HC11.The clock system in the MC6801 patent (4266270) has three clocks. I have 
verified that t here are indeed three clocks running through the schematic. 

3) the address bus internally is split into two separate busses, A0 -A7 and A8 to A15. Also 
As the title to patent 4266270 suggests there are two seaprate 8 bit data busses in the 
MC6801/3 processor. When I pulled patent 4200912 and looked at the figures #1 and #2 my 
eyes nearly popped out of my sockets. Here it was nearly indentical to the MC6809. Low 
and behold t his patent depicts a MC6809 like processor with two internal 8 bit data busses 
that has a switch to make the two a single 8 bit buss. The main scope of patent 4200912 
was a processor having multile IRQ processors. The embodied processor has the IRQ and 
FIRQ inputs. The registers include the USP,  Y pointer register, and the Direct Page 
Register. Plus a few others. 

I have started writing a blurb on this and hope to put it up on my webpage soon. This was 
neat stuff and by pouring through about ten to 15 patents,I came away with a greater 
understanding of what thought processes went through during the design of the MC6809. 
Also a greater  understanding of several of Motorola's processors and how they function 
internally. One t hing that I do understand better is how fast the processor can be clocked. 
Best I could figure is about 6 MHz and that is pushingit real hard. Three MHz is most likely 
the max for most parts. To many levels of logic gates to ripple through to get much faster. 

more later



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