Strong Pull Up for Parasitic Power mode on 1-W
Archiver
Posts: 46,084
The good news is your Stamp can provide the required current. The
bad news is, according to the DS1920 spec, the strong pullup must be
available within 10 microseconds (usec) of issuing the temperature
conversion command. It'll take your BS2p at least 40 usec to execute
a HIGH instruction. Same problem if you use the MOSFET, since you'd
have to turn it on with a similar instruction.
The other good news is I've had great success with the DS1920
without using a strong pull-up. Depending on your envrironment, you
may find that simply using a stiffer pullup resistor, such as 1.5K,
will work fine. Dallas Semi recommends routinely using a 1.5K
versus 4.7K pullup resistor in their literature.
One last thing--if you've got any other 1-wire devices which may
generate an out-of-limits/alarm/interrupt signal, use great care in
setting the Stamp's pin high, since these signals can take the bus
low unexpectedly.
Regards,
Steve
longbow42 wrote:
> I just recieved my new I-Button DS1920 temperature chip and am
> starting to experiment. Data sheets suggest a strong pull up on the
> data pin during temp conversion...
bad news is, according to the DS1920 spec, the strong pullup must be
available within 10 microseconds (usec) of issuing the temperature
conversion command. It'll take your BS2p at least 40 usec to execute
a HIGH instruction. Same problem if you use the MOSFET, since you'd
have to turn it on with a similar instruction.
The other good news is I've had great success with the DS1920
without using a strong pull-up. Depending on your envrironment, you
may find that simply using a stiffer pullup resistor, such as 1.5K,
will work fine. Dallas Semi recommends routinely using a 1.5K
versus 4.7K pullup resistor in their literature.
One last thing--if you've got any other 1-wire devices which may
generate an out-of-limits/alarm/interrupt signal, use great care in
setting the Stamp's pin high, since these signals can take the bus
low unexpectedly.
Regards,
Steve
longbow42 wrote:
> I just recieved my new I-Button DS1920 temperature chip and am
> starting to experiment. Data sheets suggest a strong pull up on the
> data pin during temp conversion...