ledc duty_resolution seems to be fixed at 13bit (GIT8266O-684)

[simonliu009]

Environment

• Development Kit: NodeMcu-8266 (2891) V1.2 made by Ai-thinker
• IDF version (git rev-parse --short HEAD to get the commit id.):
  08e225d (master branch)
• Development Env: make
• Operating System: Ubuntu in WSL
• Power Supply: USB

Problem Description

• It seems that ledc duty_resolution is fixed at 13 bit. I tried LEDC_TIMER_8_BIT/LEDC_TIMER_10_BIT/LEDC_TIMER_13_BIT for duty_resolution at 20KHz/25KHz frequency, to get 50% duty cycle I have to set duty to 4096( no matter resolution is 8bit or 10bit or 13bit), and to get 100% duty(almost 100%) I have to set duty to 8191( no matter resolution is 8bit or 10bit or 13bit). But 4096@50% duty cycle is supposed to be the duty of 13bit resolution, is that correct?
• I can't get actually 100% duty cycle in ledc by setting duty to (2**duty_resolution -1), there is still a sag by the end of each cycle.

Both release/v3.3 and master branch with latest commit have this issue. I didn't check v3.4 but I guess the result should be the same.

• 不论我怎么设置duty_resolution(我尝试了LEDC_TIMER_8_BIT/LEDC_TIMER_10_BIT/LEDC_TIMER_13_BIT)，只有将duty设置为4096才能 达到50%的duty cycle，duty设为8191才能达到100%（实际上是接近100%），频率也尝试了20KHz/25KHz，结果都一样。
• 将duty 设置为 (2**duty_resolution -1)也无法实际上达到真正的100% duty cycle，每个周期末尾还是有一个波形的跌落。

Expected Behavior

To get 50% duty cycle at 20KHz:

• duty should be 4096 with 13 bits duty_resolution
• duty should be 512 with 10 bits duty_resolution
• duty should be 128 with 8 bits duty resolution

Actual Behavior

To get 50% duty cycle at 20KHz:

• duty is 4096 with 13 bits duty_resolution -- which is correct
• duty has to be 4096 with 10 bits duty_resolution -- which is wrong?
• duty has to be 4096 with 8 bits duty resolution -- which is wrong?

** There should be a relationship between frequency and duty_resolution but this is a more complicated discussion in the case of ESP8266 because it involves ledc_clk_src, I might post my questions about frequency/duty_resolution on espressif esp8266 forum later. **

Code to reproduce this issue

The following code is based on ledc example from RTOS SDK which I used to reproduce this issue.
test code on gist

Debug Logs

1. LEDC fade to duty = 8192, duty resolution = 8, frequency = 20000
I (36333875) ledc: channel_num = 0 | duty = 49; duty_p = 24 | step_duty = 0 | step_01duty = 0 | step_001duty = 8

2. LEDC set to duty = 4096, duty resolution = 8, frequency = 20000
I (36339877) ledc: channel_num = 0 | duty = 24; duty_p = 49 | step_duty = 25;

Other items if possible

screen shots from my USB scope