Soft pwm enhancement #6015
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Soft pwm enhancement #6015
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A 128 step PWM has 127 intervals (0/127 ... 127/127 duty). Currently, a PWM setting of 1/127 is active for 2/128, i.e. double the expected time, or, in general n+1/128 instead of n/127. Fixes issue#6003. Signed-off-by: Stefan Brüns <[email protected]>
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Does the dithering add any performance overhead? |
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The performance overhead is very low - AFAICT it adds one machine instruction per PWM channel on each timer interrupt (1kHz). |
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Small correction, it is three instructions, but of course only run once every PWM cycle, i.e. 16 times a second for SCALE 1. |
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Device: atmega1284p, TinyBoy2 (using U8glib) FW size without SR applied: With SR: |
thinkyhead
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Mar 18, 2017
Marlin/temperature.cpp
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| const uint8_t pwm_mask = _BV(SOFT_PWM_SCALE) - 1; | ||
| #else | ||
| const uint8_t pwm_mask = 0; | ||
| #endif |
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The const lines should be indented. This is also good:
const uint8_t pwm_mask = 0
#if ENABLED(SOFT_PWM_DITHER)
+ _BV(SOFT_PWM_SCALE) - 1
#endif
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Indented the declaration, fixed the commit subject (s/is/if)
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If dithering is enabled, the remainder of the soft_pwm_X duty value at turnoff time is added to the next cycle. If e.g. the duty is set to 9 and SCALE is set to 2, the PWM will be active for 8 counts for 3 cycles and 12 counts on each fourth cycle, i.e. the average is 9 cycles. This compensates the resolution loss at higher scales and allows running fans with SOFT_PWM with significantly reduced noise. Signed-off-by: Stefan Brüns <[email protected]>
The compiler is not able to reuse the value of pwm_count, but reloads it on every evaluation, if is stored in a static variable, as it cannot prove it will be unchanged. A variable with local scope may not be modified from the outside, so its value can be reused. Doing so reduces text size and instruction count. Signed-off-by: Stefan Brüns <[email protected]>
After wraparound, pwm_count <= pwm_mask holds, thus soft_pwm_X <= pwm_count guarantees soft_pwm_X < pwm_mask is true, and the heater will be switched off in the first branch. Do not evaluate the pwm conditions a second time, this reduces the instruction count (4 instructions per PWM) and text size (6 byte). Signed-off-by: Stefan Brüns <[email protected]>
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Superseded by #6100 |
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Includes fix for #6003: For low duty cyles, the actual duty cycle is larger than wanted
Supersedes #5997
Dithering allows to mitigate the resolution loss caused by the scaling.