用28027外部时钟输入口计数，每次输入到第7个后才开始计数，即输入第7个脉冲，读得的值是1， 输入第8个脉冲，读得的值是2，依此....
28035也是这样。
附初化代码：
EALLOW;
GpioCtrlRegs.GPAMUX2.bit.GPIO19=0;
GpioCtrlRegs.GPADIR.bit.GPIO19=0;
GpioCtrlRegs.GPAPUD.bit.GPIO19=0;
GpioCtrlRegs.GPAQSEL2.bit.GPIO19=0x2;
SysCtrlRegs.XCLK.bit.XCLKINSEL = 1;
SysCtrlRegs.CLKCTL.bit.XCLKINOFF = 0;
SysCtrlRegs.CLKCTL.bit.XTALOSCOFF = 1;
SysCtrlRegs.CLKCTL.bit.TMR2CLKSRCSEL = 1;
SysCtrlRegs.CLKCTL.bit.TMR2CLKPRESCALE = 0;
EDIS;
EALLOW;
CpuTimer2Regs.PRD.all=0x0000FFFF;
CpuTimer2Regs.TPR.all=0;
CpuTimer2Regs.TPRH.all=0;
CpuTimer2Regs.TCR.bit.TSS=1; // stop
CpuTimer2Regs.TCR.bit.TRB=1; // reload (w1_clear)
CpuTimer2Regs.TCR.bit.TIE=0; // disable interrupt
CpuTimer2Regs.TCR.bit.FREE=0; // Emulation Mode: hard stop
CpuTimer2Regs.TCR.bit.SOFT=0; //
EDIS;