I did a couple of generations of control systems based on 8051 detivatives. These had a bit of ROM - 8K, usually - but only 128 bytes of RAM. The stack used that 128 bytes, and so did your variables. Pretty darn tight fit for interrupt driven code, which needs to use some stack.
I did the If VI Were IX guitar robots on the 8051. It handled MIDI input over the serial port in interrupts, and it had a timer driver interrupt that we used to both update our servo loop and also to derive the pulse chain used to control position on the pluckers - the shaft with a plastic pick or nib that plucks the string when rotated properly.
We put 2 on each shaft - as I said, a pick and a nub, and added support for a MIDI mode switch to select between the two. Based on the requested velocity in MIDI note on commands received from the serial port, we would set the PWM parameters higher (mostly on) for high velocities and lower (mostly off) for low velocities. To make the PWM average out and not be too jerky and noisy, we need to update it as fast as we possibly can. Bear in mind that our 8051 used a 4 MHz clock, and many instruction took more than 4 cycles, so we got less than 1 million instructions per second. Not much power for handling real time updates and asynchronous serial input while playing a guitar in real time.
(Old man ranting at lazy kids mode)
Micro-controller chips today usually have hardware PWM circuits, so we can just load a duty cycle number and provide a really fast MHz+ digital clock and we get a great PWM. Luxury! The 8051 I was using had no PWM hardware, so we implemented it in software using interrupts. Messier, less smooth, lots more code and a few variables in a system that had little room for either. We couldn't even get 1M instructions/sec.
Micro-controllers today also have more RAM - either built in, or external, they just don't make them with 128 bytes much any more. Luxury! (You're supposed to hear the Monty Python bit about having to walk to school uphill both ways, not like the lazy kids today; doesn't come across well in text). Clocks on modern micro-controllers are often in the gigahertz, a thousand or more times faster than the 8051, and also 32 bits wide, so each instruction handles and processes 4 times as much data as the old 8051s could.
So we had all of our local variables - assorted modes (damper tied to notes, or off to allow hammer-on and hammer-off, or on for a damped sound, etc), state (plucking has several states and we need requested and expected positions i order to include the error component in the feedback loop), limit details, channel details, note range details, and more. We also had to have enough left over in the 128 bytes to allow for the registers to be stored during an interrupt (MIDI IO) with enough room for an additional stack frame for an overlapping timer interrupt (servo and position updating).
We managed to squeeze it all in and it works fine. It helps that registers are only 8 bits and there aren't many of them, and the program counter (pushed onto the stack as the return address) is small too - not all that much needs to be pushed on the stack. The upside of little room is that you simply can't have code bloat and variables must be as simple as possible. The result is small enough that you can realistically squeeze all of the bugs out.
The If VI Were IX installation has never crashed due to software fault, and has outlived every single moving part in the system - MIDI sources had to be replaced with a solid state source, pluckers and strings replaced, yet the 8051 micro-controllers are still fine a decade later.
If I was doing this over again from scratch today, I'd probably base it on a Raspberry Pi system with gigabytes of memory and a flash hard drive with tens of gigabytes more. Luxury!
I did the If VI Were IX guitar robots on the 8051. It handled MIDI input over the serial port in interrupts, and it had a timer driver interrupt that we used to both update our servo loop and also to derive the pulse chain used to control position on the pluckers - the shaft with a plastic pick or nib that plucks the string when rotated properly.
We put 2 on each shaft - as I said, a pick and a nub, and added support for a MIDI mode switch to select between the two. Based on the requested velocity in MIDI note on commands received from the serial port, we would set the PWM parameters higher (mostly on) for high velocities and lower (mostly off) for low velocities. To make the PWM average out and not be too jerky and noisy, we need to update it as fast as we possibly can. Bear in mind that our 8051 used a 4 MHz clock, and many instruction took more than 4 cycles, so we got less than 1 million instructions per second. Not much power for handling real time updates and asynchronous serial input while playing a guitar in real time.
(Old man ranting at lazy kids mode)
Micro-controller chips today usually have hardware PWM circuits, so we can just load a duty cycle number and provide a really fast MHz+ digital clock and we get a great PWM. Luxury! The 8051 I was using had no PWM hardware, so we implemented it in software using interrupts. Messier, less smooth, lots more code and a few variables in a system that had little room for either. We couldn't even get 1M instructions/sec.
Micro-controllers today also have more RAM - either built in, or external, they just don't make them with 128 bytes much any more. Luxury! (You're supposed to hear the Monty Python bit about having to walk to school uphill both ways, not like the lazy kids today; doesn't come across well in text). Clocks on modern micro-controllers are often in the gigahertz, a thousand or more times faster than the 8051, and also 32 bits wide, so each instruction handles and processes 4 times as much data as the old 8051s could.
So we had all of our local variables - assorted modes (damper tied to notes, or off to allow hammer-on and hammer-off, or on for a damped sound, etc), state (plucking has several states and we need requested and expected positions i order to include the error component in the feedback loop), limit details, channel details, note range details, and more. We also had to have enough left over in the 128 bytes to allow for the registers to be stored during an interrupt (MIDI IO) with enough room for an additional stack frame for an overlapping timer interrupt (servo and position updating).
We managed to squeeze it all in and it works fine. It helps that registers are only 8 bits and there aren't many of them, and the program counter (pushed onto the stack as the return address) is small too - not all that much needs to be pushed on the stack. The upside of little room is that you simply can't have code bloat and variables must be as simple as possible. The result is small enough that you can realistically squeeze all of the bugs out.
The If VI Were IX installation has never crashed due to software fault, and has outlived every single moving part in the system - MIDI sources had to be replaced with a solid state source, pluckers and strings replaced, yet the 8051 micro-controllers are still fine a decade later.
If I was doing this over again from scratch today, I'd probably base it on a Raspberry Pi system with gigabytes of memory and a flash hard drive with tens of gigabytes more. Luxury!
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