qep的输入时钟是什么呢？
查看2812的数据手册6.2.2.7 Capture Unit
The input pins CAP1/2 and CAP4/5 can also be used as QEP inputs to the QEP circuit.
Two capture inputs (CAP1 and CAP2 for EVA; CAP4 and CAP5 for EVB) can be used to interface the onchip QEP circuit with a quadrature encoder pulse.
