Some understanding of high level effective and low level effective in STM32

In the process of learning STM32, we often encounter words such as "high level is effective" and "low level is effective". When we first see them, we often understand them literally, thinking that high level is effective means the effective level is high level, and low level is effective means the effective level is low level. In fact, this understanding is wrong. Let's take the output comparison channel in the STM32 timer as an example:

This picture is actually a process of generating a PWM wave. Those who don't know much about timers can refer to the relevant manual. Now let's look at the output mode controller labeled 1 in the figure. The mode here refers to the PWM mode, which means that the PWM mode can be selected by configuring the two bits OC1M of the register TIMx_CCMR1. However, regarding mode selection, there is such a sentence in the manual: When counting down, once TIMx_CNT>TIMx_CCR1, channel 1 is an invalid level (OC1REF=0), otherwise it is a valid level (OC1REF=1).

The red font tells us: when OCREF is an invalid level, it is a low level, and when OCREF is a valid level, it is a high level.

Let's look at label 2: Its function is to select the output polarity. If the CC1P bit of TIMx_CCER is 0, the high level is valid, otherwise the low level is valid. What does this high level valid mean? For the selector, the high level is valid. If OCREF is high, the gate of the selector will open, allowing the signal to enter and "activate" the selector. After that, the selector will generate a signal to activate the subsequent output enable circuit to drive OC1, so that it generates a valid level on OC1 (that is, a high level is generated on OC1).

Through the above analysis, we can find that high level valid means that if the signal is at a high level, it is valid for the corresponding controller and can activate the relevant functions of the controller.

Summarize a few key words: Effective level: high level

                         Invalid level: low level

                         High level is valid: high level can enable the corresponding function

                         Low level is effective: Low level can enable the corresponding function