The TRS-80 Model 100 was released in 1983, featuring an 80C85 CPU that can run at 5 MHz, but only runs at a hair under 2.5 MHz, due to 1:2 divider on the input clock. Why cut the speed in half? It has a lot to do with the focus of the M100 on being a portable device with low power usage. Since the CPU can run at 5 MHz and modding these old systems is a thing, we got a ready-made solution for the TRS-80 M100, as demonstrated by [Ken] in a recent video using one of his ‘daily driver’ M100s.
This uses the board design from the [Bitchin100] website, along with the M100 ROM image, as one does not simply increase the CPU clock on these old CPUs. The issue is namely that along with the CPU clock, connected components on the CPU bus now have to also run at those speeds, and deal with much faster access speed requirements. This is why beyond the mod board that piggybacks on top of the MPU package, it’s also necessary to replace the system ROM chip (600 ns) with a much faster one, like the Atmel AT27C256R (45 ns), which of course requires another carrier board to deal with incompatible pinouts.
After this, we find that the system RAM also has access speed issues, due to the power-saving logic that’s part of the original board. The mod board deals with this by bypassing part of the logic and triggering the power-up signal (A*) much sooner, so that even the original base system RAM can scrape by. This then only leaves the UART which objects to being fed a 5 MHz signal, as this ruins its clocking ability. For this, the mod board also has a fix: a 2.45 MHz output pad.
With the mod fully installed, and the model 100 closed up again, it is confirmed that the system is indeed faster. Unfortunately, it also breaks some games, like Starblaze 100, but since you can switch between 2.5 and 5 MHz mode on the command line, this may not be such a dealbreaker.
Thanks to [Stephen Walters] for the tip.
Tagged m100, model 100, TRS-80 model 100
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