On Tue, 10 Jun 2014, Gerrit Heitsch wrote: > The problem is, as long as RESET is +10V, the datasheet claims ALL > fetches are done via Port PC. So you cannot run a program from RAM > until you return RESET to +5V. Catch-22... Actually, it claims that all *memory* fetches are made from Port PC. Whatever "memory" means here. Could be the entire memory range, given that it's memory-mapped I/O, or just ROM (F)800-(F)FFF. With respect to Page 3, I wouldn't be surprised if it is only the latter. Chip-wise, it would even be easier in hardware as only the following changes would need to take place (in pseudo VHDL): -- this is the actual overhead TEST <= '1' when rst=10V else '0'; nTEST <= not(TEST); -- let TEST disable the ROM buffer/driver -- difference: just a wire, i.e. TEST instead of '0' ROM_CS <= TEST when a(11)='1' else '1'; -- let TEST disable the pin driver -- again, difference is only a wire for i in 0 to 7 generate PC_pin(i) <= nTEST when PC_reg(i)='1' else '0'; end generate; That's it. An biased transistor for getting nTEST='0' upon rst=10V, '1' else, an extra transistor for inverting it, and a little bit of wiring. Anything else would add complexity: if *any* access to $000-$fff were to be routed through PCx, then also muting of RAM, PAx, PBx, PCx, PDx etc. would apply, which not only means routing TEST/nTEST all over the chip but also more terms, such as: RAM_CS <= '0' when a(11 downto 7)="00000" and rw='0' -- RAM write always else TEST when a(11 downto 7)="00000" and rw='1' -- RAM read only if not TEST else '1'; instead of just RAM_CS <= '0' when a(11 downto 7)="00000" else '1'; Likewise, PA/PB/PC/PD port reads would need to be muted in order to avoid collision with the PCx input. Since the overall efforts in hardware are significantly lower for the first case, i.e. only ROM (read) access is appearing at PCx, and given that this simpler solution even makes debugging easier (as jumping to 0000 would now fetch data from RAM, giving easy software control over the ports), I wouldn't see the benefits of the "any access will come from PCx", but maybe I'm missing something. Rainer Message was sent through the cbm-hackers mailing listReceived on 2014-06-10 23:00:03
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