We have an issue on what can we called a bad design device… The devise is a led alarm manager on 1-wire bus.
It uses 36 bidirectionnal leds, 10 normal leds, 7 ULN2004AD and 6 DS2408.
At startup, some leds are ON while this shouldn’t be the case. May anyone can give me some advice on how to solve that?
It’s occuring on the same 2 leds not all the time but after cold startup. Switching it off/on after initial start and the system works. We have build two board with the same phenomenon (but not the same leds)
I tried to pull up RSTZ without success…
The datasheet for the DS2408 shows this.
Power-up State of P0 to P7
[em]When the DS2408 powers up, the state of the I/O pins P0 to P7 is indeterminate. This behavior may not
be acceptable for some applications. To ensure that P0 to P7 power up in the “off” state, it is necessary to
have a suitable power-on-reset circuit, such as the DS1811, or a supervisor IC connected to the RSTZ pin. [/em]
You can’t just pull up RSTZ, you need to put a reset on this so that it remains LOW on power up. They show how to do this in the datasheet.
Here is a possible solution. Put a PULL DOWN on this pin and connect to a GPIO that is configured initially as an INPUT. On power up the pin will be LOW and after the CPU is ready, set the GPIO as output and set it high. Your DS2408 is now ready to go and your outputs should remain off.