更精确的计时

一般使用Timer物件，没有办法精确的算到微秒，(每秒目更新18.2次)，若要算到
微秒，则使用GetTickCount ，它传回Windows启动後到目前为止所经过的时间，
传回值以微秒为单位。

Private Declare Function GetTickCount Lib "kernel32" Alias _
       "GetTickCount" () As Long

Private CanContinue as Boolean

Private Sub Command1_click()
Dim i as Long
Dim j as Long
i = GetTickCount()
CanContinue = True
Do While CanContinue
   j = GetTickCount()
   if j - i > 50 Then
      Debug.Print "已过50微秒"
      i = j
   End If
   DoEvents
Loop
End Sub

Private Sub Command2_Click()
  CanContinue =  False
End Sub