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