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StefanBruens / ESP8266_new_pwm

This is a drop-in replacement for the ESP8266 SDK PWM
GNU General Public License v2.0
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Compat Mode and NMI Timer #7

Closed kaeferfreund closed 7 years ago

kaeferfreund commented 7 years ago

Hi,

just wondering about two things:

  1. why exactly 200ns for the "non compat mode"?
  2. does compat mode work when the NMI Timer is used?

Thx in advance

StefanBruens commented 7 years ago

Each instruction at 80MHz CPU clock takes 12.5ns, so 200ns is 16 clocks, i.e. 16 instructions in best case. Clearing and setting any GPIOs has a latency of 6 clocks each (CPU design limitation), which leaves 4 instructions for fetching the settings of the next PWM "step". Also the timer is running in "Divide by 16" mode, i.e. 200ns.

A finer granularity is not useful - there is some spurious jitter of the same magnitude, and if you e.g. switch some mosfets, rise/fall times will vary by several 10ns depending on your power supply stability.

NMI and compat mode are unrelated, but you should avoid compat mode if possible, as it lowers the resolution to 1000ns.