On Sat, Nov 12, 2022 at 12:18 PM Chuck Guzis via cctalk <cctalk@classiccmp.org> wrote: > On 11/12/22 02:28, Tony Duell via cctalk wrote: > > ... This is the sort of > > thing I'd do with a couple of transistors or an NE555 depending on > > which turned up in the junk box first. > > One thing that a small MCU has over a 555 is that it can be programmed > once and you can be assured of its frequency stability. No fooling with > pots and caps to get the thing to work the way you'd like.
Yes, that, plus since many products have a hardware team and a firmware/software team, the hardware can be designed to general requirements and sent out for manufacture while the software team has time to write the firmware (and make changes long after the hardware is set). One of the first times I encountered this was stripping some old emergency exit lights for parts c. 2008. The switching supply had an 8-pin PIC for the oscillator instead of a 555. Yes, a 555 could have done it, but the PIC didn't need any external components to set the frequency, components that can drift with age, and components that take up board space. Even if the 555 and MCU were identical in cost for the IC, the MCU was cheaper because of the smaller footprint. Additionally, the designers had some flexibility. To change the frequency with a 555 after manufacture is an expensive proposition. With an MCU, if it's flashable in-circuit (clip or possibly programming pads near/at the MCU), then one can change the behavior without melting any metal or purchasing components. While one may never need to change the frequency of a SMPSU oscillator after initial design, there are plenty of products where it's handy that the hardware guys can say "here's an output that can go from 1/10Hz to 20Khz - what do you want it to be?" and not worry about design limitations, just set the frequency in the firmware and it does that. You can build some generic hardware (X inputs, Y outputs with Z mA current drive) and fine tune things later, or have variations on what the inputs mean and not have to change the PCB. Yeah, the 555 is extremely simple and is well known and is fairly cheap, simple MCUs are simple (and cheap) even if they aren't 100% deterministic like a chip with 20-30 transistors. There's economic advantage in flexibility. -ethan
