3.5.0 beta 3 minimum PWM frequency

PascalM

Hi,
I upgraded my Duet 3 6HC from 3.5.0 beta 2 to beta 3 and found out that my bed heater turned on permamently.
I have configured the bed heater output to 1Hz PWM and up until now it worked flawlessly. The output controls a 230V solid state relay that switches on zero-crossings of the 50Hz mains supply, therefore I want a slow enough PWM frequency to have reasonable resolution of bed heater power.

I tested 2Hz and the problem remains, at 3Hz the output seems to be working again as intended.
Therefore, as a workaround, 3Hz is ok, but my effective PWM resolution is really bad.
Fortunately, at 60°C bed temperature, its still acurate enough to have the bed temperature rock solid at 60.0°C even without fluctuations to 60.1 or 59.9. Might get worse at lower temperatures.

I guess it is a result of the changed PWM resolution in beta 3
("[Duet 3 MB6HC] [Duet 3 MB6XD] [Duet 2] [Duet Maestro] PWM resolution when using outputs connected to the PWM peripheral is increased")
Is it possible to have an option to enable/disable the higher PWM precision, or automatically use reduced resolution PWM, when requested PWM is too low?

At least, for safety, I would advise to disable the output and give an error if a too low frequency is requested. I protected my bed with a thermal switch and a thermal fuse, but it might not be on every printer.

My configuration of the heater:
In sys config.g

M308 S0 P"temp0" Y"thermistor" T100000 B4583 C1.022833e-7 
M950 H0 C"out0" T0 Q1
M307 H0 B0 S1.00
M140 H0
M143 H0 S130

Override with the tuning at 60°C for the filament I tested with, in filaments config.g

M307 H0 R0.15 K0.161:0.000 D9.31 E1.35 S1.00 B0

dc42

@PascalM thanks for reporting this. I'll try to fix it in 3.5beta4.

https://github.com/Duet3D/RepRapFirmware/issues/854

dc42 created this issue in Duet3D/RepRapFirmware

closed New PWM code in RRF 3.5 doesn't allow PWM frequencies as low as 3.4 did #854

T3P3Tony

@PascalM please can you test to see if this issue is resolved with the pre release here:
https://www.dropbox.com/sh/in0t2j68ic66oa8/AADv_6WzRy07H6MhBOrkDt23a?dl=0

PascalM

@T3P3Tony I testet with the pre release. It is working at 1Hz again.

dc42

@PascalM thanks for confirming.