使用eCAP进行BLDC霍尔测速，测速结果跳变很大怎么回事

我在使用F28030做BLDC速度控制，使用eCAP管脚获取一个霍尔管脚(三相)的信号。用示波器观测速度为600转Rpm左右(正负15转)，可是使用eCAP测试结果差别很大，一会800，一会600，跳变严重。文件配置如下：
void InitECapture()
[
ECap1Regs.ECEINT.all = 0x0000; // Disable all capture interrupts
ECap1Regs.ECCLR.all = 0xFFFF; // Clear all CAP interrupt flags
ECap1Regs.ECCTL1.bit.CAPLDEN = 0; // Disable CAP1-CAP4 register loads
ECap1Regs.ECCTL2.bit.TSCTRSTOP = 0; // Make sure the counter is stopped

// Configure peripheral registers
ECap1Regs.ECCTL2.bit.CONT_ONESHT =0; // One-shot
ECap1Regs.ECCTL2.bit.STOP_WRAP = 3; // Stop at 4 events
ECap1Regs.ECCTL1.bit.CAP1POL = 1; // Falling edge
ECap1Regs.ECCTL1.bit.CAP2POL = 0; // edge
ECap1Regs.ECCTL1.bit.CAP3POL = 1; // Falling edge
ECap1Regs.ECCTL1.bit.CAP4POL = 0; // edge
ECap1Regs.ECCTL1.bit.CTRRST1 = 1; // Difference operation
ECap1Regs.ECCTL1.bit.CTRRST2 = 1; // Difference operation
ECap1Regs.ECCTL1.bit.CTRRST3 = 1; // Difference operation
ECap1Regs.ECCTL1.bit.CTRRST4 = 1; // Difference operation

ECap1Regs.ECCTL2.bit.SYNCI_EN = 0; // Enable sync in
ECap1Regs.ECCTL2.bit.SYNCO_SEL = 0; // Pass through
ECap1Regs.ECCTL2.bit.TSCTRSTOP = 1; // Start Counter
ECap1Regs.ECCTL2.bit.REARM = 1; // arm one-shot
ECap1Regs.ECCTL1.bit.CAPLDEN = 1; // Enable CAP1-CAP4 register loads
ECap1Regs.ECEINT.bit.CEVT4 = 1; // 4 events = interrupt
]

interrupt void ECap1_isr(void)
[

speed();
ECap1Regs.ECCLR.bit.CEVT4 = 1;
ECap1Regs.ECCLR.bit.INT = 1;
ECap1Regs.ECCTL2.bit.REARM = 1;//持续捕捉外部信号
// Acknowledge this interrupt to receive more interrupts from group 4
PieCtrlRegs.PIEACK.all = PIEACK_GROUP4;
]
void speed(void)
[
T1=ECap1Regs.CAP1;
T2=ECap1Regs.CAP2;
T3=ECap1Regs.CAP3;
T4=ECap1Regs.CAP4;
T=T1+T2+T3+T4;
n=3600000000.00/T;
if(n>=550&&n<=700)
[
LightLED1();
]
else
[
ShutLED1();
]
]