USB模块干扰输入捕获测量

您好，我需要您的帮助与一个奇怪的问题：我有一个非常简单的设置与PIC24FJ256GB206，它测量风扇的转速使用IC模块。此外，我有（未修改）微芯片USB堆栈（V2.9J）作为CDC设备在中断模式下运行（不轮询）。当我拔下USB PORT，我经常从IC模块得到错误的测量结果。它们并不是完全错误的，但是两个脉冲之间的测量周期大约是实际值的1.5倍。我设置IC模块的方式是每秒上升沿产生一个中断。在IC中断期间，从FIFO中的两个定时器值计算周期（参见下面的代码）。很简单，很基础。工作完全稳定24/7直到USB电缆被拔掉。现在我甚至不知道从哪里开始寻找这个问题。USB模块实际上是如何干扰IC模块的？它是如何在测量中产生延迟的？显然，必须对USB堆栈做些什么；如果我禁用了附加/分离检测，问题就不会出现（当然，USB模块是不可用的）。有什么想法吗？感谢和问候，丹尼尔



      以下为原文

    Hello all,
I need your help with a strange problem:
I have a quite simple setup with a PIC24FJ256GB206 which measures the rpm of a fan using the IC module.
In addition, I have the (unmodified) Microchip USB stack (v2.9j) running as a CDC device in interrupt mode (not polling).

When I unplug the USB port, I often get wrong measurements from the IC module. They are not totally wrong, but the measured period between two pulses is approximately 1.5x the actual value.
The way that I have set up the IC module is that an interrupt is generated every second rising edge. During an IC interrupt the period is calculated from the two timer values in the FIFO (see code below). Very simple, very basic. Works perfectly stable 24/7 until the USB cable gets unplugged.

Now I don't even know where to start looking for the issue. How can the USB module actually interfere with the IC module? How does it generate the latency in the measurement I see here?
It clearly has to do something with the USB stack; if I disable the attach / detach detection, the issue does not show up (but of course USB module is not usable).

Any ideas?
Thanks & best regards,
daniel


// set up IC1 module for rpm capturing (uses TIMER4)
IC1CON1 = 0;
IC1CON2 = 0;
IC1CON1bits.ICTSEL = 0b10; // selects timer 4
IC1CON1bits.ICI = 0b01; // interrupt on every 2nd capture event
IC1CON1bits.ICM = 3; // simple capture mode: capture on every rising edge
IPC0bits.IC1IP = 6; // Setup Input Capture 1 interrupt with high priority
IFS0bits.IC1IF = 0; // Clear the IC1 interrupt status flag
IEC0bits.IC1IE = 1; // Enable IC1 interrupts

// clear capture buffer
while(IC1CON1bits.ICBNE)
{
  j = IC1BUF;
}

.
.
.

void __attribute__ ((__interrupt__)) _IC1Interrupt(void)
{
    WORD per;
    per = IC1BUF;       // last
    fan[0].period = IC1BUF - per;       // new - last            
    fan[0].timeout = 0;
   
    IFS0bits.IC1IF = 0; // Reset respective interrupt flag
}