What is the distinction between Output Compare and Pulse-Width Modulation functions?

Question

I am brushing up on microcontrollers, using the STM32 series. (Specifically using the STM32F072BDISCOVERY board).

I am having some trouble understanding the use of timers and their various modes. Specifically, there are a lot of options for "Output Compare" (OC) modes, and other options for "PWM" modes. The RM0091 Reference manual is written as if they are two distinct, alternate modes of operation. I am working with the new HAL drivers as well, and there are different APIs to use OC vs PWM.

However, it looks to me like PWM is basically a subset/use case of the Output Compare capability, where the timer channel is configured to directly drive a hardware output.

Is there a firm distinction between OC and PWM modes that I am missing? In what way is PWM waveform generation not an Output Compare function?

Answer 1

The question is old but I was wondering the same and started digging into the topic.

In an STM32 micro, not all timers implement every function. My post is based on the STM32F030 Timer 1, which is I believe the most feature packed 4 channel implementation (or at least it was a few years ago).

The timer basically acts as a counter, with various options to where the counter clock comes from, what resets it, what the period is and the counter direction. This is the base for all extra features it implements, the HAL driver refers to it as Time Base. By itself, the counter function is not related to the timer channels.

There are 1-6 channels implemented in STM32 timers. These channels can be configured either independently, or in some case, in pairs (for functions like quadrature encoder mode). A channel can be configured as either input capture, or output compare. The input capture "listens to" some event and saves the counter of the time base in the CCRx registers. The output compare compares the counter register to a set value, given in the CCRx registers.

All the simple IC/OC modes and the extra functions, like PWM input, output, Hall effect sensor interfacing, are built on top of these two modes, their respective option bits act on the various input/output multiplexers and, regarding to OC mode basically tell the hardware what action to take based on the comparator outputs (CNT = CCRx, CNT > CCRx). In this case, PWM mode lets the output mode controller return to the CNT <= CCRx state when the counter register resets (more specifically, the update event), where as in the other OC modes the output mode controller ignores this signal and can be reset by hand, or by an external signal. The output signal is the OCxREF signal which then passes through some more hardware before it gets to the output pins. If it gets output at all, because you are allowed to not connect the timer to the output pins.

The STM32 timers are complicated. They have many logical blocks and a ton of configuration registers/bits so I may have missed something or completely misread something. Please feel free to correct me.