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???
07/07/03 10:57
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 		 #50199 - RE: 89S8252 and RTC 72421
Responding to: ???'s previous message
I am currently working with the Philips PCF8563 on an I2C bus. It has a simple Li non-rechargable battery with capacity to power the chip for almost 3 years of shelf life.

Peter, there is another very good argument of using a serial RTC chip too. Since the I2C bus is an open-drain bus with pullup resistors it is very effective to power the RTC from the on-board +5V through a diode and there is another diode from the Li battery. This works good in this instance because the chip has no need for active high outputs or multiple input thresholds that could be affected by the Vcc of the RTC being at 4.3 -> 4.5 volts during normal operation. We have used a BAT54 type diode so it as lower drop than an normal diode as well.


Again I would tell Robert that I would be hard pressed to recommend the diode solution in the VCC of the parallel interfaced RTC chip.


Here is how we have the battery and VCC connected to the PCF8563 chip.

Save This Code
   +5V --+---+------+          +----------+
         |   |      |  BAT54C  |          |
         |   |     ---        ---         |
         X   X     \ /        \ /         | ++
      1K X   X    '---,      '---,      -----
         X   X 1K   |          |         ===   3.6V
         |   |      +-----0----+        -----  Li Cell
         |   |            |8             ===
         |   |     +------------+         |
         |   |   6 |   VDD      | 3       |
 SCL-----o---------|SCL     -INT|---      |
             |   5 |            | 7       |
 SDA---------o-----|SDA     -OUT|---      |
                 1 |            |         |
    +--)|-----o----|OSC1        |         |
    |  18pf   |    |            |         |
    |        ---   |            |         |
    |  32768 [ ]   |  PCF8563   |         |
    |    Hz  ---   |    RTC     |         |
    |         |  2 |            | 4       |
    o--)|-----o----|OSC2     GND|---------o
    |  22pf        |            |         |
    |              +------------+         |
    |                                     |
  -----                                 -----
   ---                                   ---
    -                                     -


Michael Karas


List of 11 messages in thread
TopicAuthorDate
89S8252 and RTC 72421            01/01/70 00:00      
   RE: 89S8252 and RTC 72421            01/01/70 00:00      
      RE: 89S8252 and RTC 72421            01/01/70 00:00      
   RE: 89S8252 and RTC 72421            01/01/70 00:00      
      RE: 89S8252 and RTC 72421            01/01/70 00:00      
         RE: 89S8252 and RTC 72421 / Try Again            01/01/70 00:00      
            RE: 89S8252 and RTC 72421 / Try Again            01/01/70 00:00      
            RE: 89S8252 and RTC 72421 / Try Again            01/01/70 00:00      
      RE: 89S8252 and RTC 72421            01/01/70 00:00      
   RE: 89S8252 and RTC 72421            01/01/70 00:00      
      RE: 89S8252 and RTC 72421            01/01/70 00:00      

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