Lpc2000

Hi,
          I am using LPC2136.I am facing some RTC related problems in 
few boards. I am writing some value in RTC on power on but it is not 
incrementing. 
         This type of problem is not in all board but few of them.Some 
board work without any problem.
          I also observed that when i use jlink for debug ,value of 
RTC register(SEC,MIN etc) are incrementing ,but these increment are 
slow. When i come out of debug & send data on serial port ,i have 
getting 0 data continously.
         I have also check VBAT voltage which found ok.
         Please help me ,if any one can.
     
Regards,
Pandurang S.