That sure does sound reasonable. I love all the tricks and hacks old designers used to do like that.
bkw On Sat, Jun 18, 2022, 9:46 AM Stephen Adolph <twospru...@gmail.com> wrote: > I've often wondered about the purpose of the A* signal on the M100. > > A* is used to control the internal RAM. It drives the timing for the RAM > chip selects. Also, since the RAM are wired with OE grounded, whenever A* > is high, the RAM will try to immediately output data, unless the processor > is writing data to it. > > A* is logically the NAND of /WR and /RD. A* is '1' whenever a read or > write is happening. So the ram chip selects never get enabled until well > into the processor cycle, far later than it could be based on IO/M and > address lines. > To me this has always been a bit peculiar. Chip selecting, and the > function of reading or writing have separate timings and can be done > separately. > > Also, I think A* is used to enable "external RAM" on the M100. I think > that is how the PG Designs ram bank products work for example - by > controlling A*. > > So, why did the M100 designers choose to delay the chip select for the RAM? > > As I have been playing around with 5MHz hacking on these things I have > found that, in order to speed up the computer, one has to fiddle with A*. > Generally, A* has to be enabled EARLIER in the processor cycle in order to > give the RAM time to respond at the higher speed. > > I've noticed that the computer power consumption is quite sensitive to the > actual timing of A*. The earlier I enable A*, the higher the current. > > Here's some numbers: > At 2.5 MHz (stock speed) the RAM in my M100 consumes about 2.3 mA out of > the 50 mA of the machine overall. This is with a "stock" A* signal. > > In the extreme case of setting A* permanently on (hard wired to 5V) , at > 5MHz the RAM current shoots up to 20mA! and the M100 is consuming 85 mA. > (not great right?) > > If I do a bit of work, I can generate an A* signal that minimizes RAM > current to 4.3 mA while operating at 5MHz. Much better! this keeps the > overall M100 power to <70mA @5MHz. > > Re-reading various datasheets, and thinking about memory power ... I think > the M100 hardware designers were using A* to significantly limit the amount > of time that the RAM chips were enabled. By doing so, they dramatically > reduced the overall memory power. This of course makes sense for a battery > powered computer. > > This was an interesting ah-hah. In retrospect, it makes perfect sense > once you realize that "on" time is driven by chip-select and that drives > power. > > And the result of all this twiddling is that my 5MHz hacks are quite a bit > lower power. > > I have some new PCBs on the way. These boards piggy back on the CPU, and > provide the faster clock. Now they also generate a new A* signal for the > RAM. > > In terms of computer power, I expect the following. > 1) for a FIXED (ie permanent) 5MHz conversion, the increase in current > should be 18mA > 2) for a SWITCHABLE conversion, the increase in current should be 3mA in > 2.5M mode and 21 mA in 5M mode. > > Standard system RAM is usable in both T102 and M100. > The only memory part that needs to be changed is the M100 ROM, which is > very very slow. So 5MHz must have an EPROM conversion or similar. > > > > > > > >
