18F4420 and 1-Wire

[Amoque]

Yes, of course.

I forgot the pull-up resistor (4K7) when designing the board. I had intended to add it at the "plug" end of the cable, then forgot that as well; without this pull-up the "Waitloop" never exits. Once the hardware was correct, the temperature readings were suspiciously low (I suspect for the reasons Alain suggests). "Room temperature" reading was 62F, but my analog thermometer read closer to 70F; placing the sensor under my arm should have demonstrated somewhere near 98F, but the measured value was only 85F. Bruce's (Reynold's Electronics) conversion routine gives more accurate results:

Dummy = Raw_Temperature * 1125
Raw_Temperature = DIV32 100
Raw_Temperature = Raw_Temperature +3200
Temp_in_F = Raw_Temperature

While I cannot verify that this formula yields precise results, room temperature reading of 69.14 was much closer to thermometer reading 0f 70; and under arm reading rose to 97+.

[Dave]

4K7 seems to be a bit on the high side, I have always used 2k2 for my pullups. Maybe a reason?

[Amoque]

The datasheet calls for a "strong" pull-up and recommends 4K7; I searched the internet and saw others having success with this value. I have no knowledge or experience that suggests a better value, so I tried it and it seems to work well.

[Acetronics2]

4K7 seems to be a bit on the high side, I have always used 2k2 for my pullups. Maybe a reason?

a very good reason is the cable length between the probe and the PCB ....

Alain

[Demon]

I haven't tested with long wires yet:
http://www.picbasic.co.uk/forum/showthread.php?t=17282
but 4K7 worked at 115,200 at 64MHz in preliminary tests.

(Diagram in post #1 is not updated, only 2 resistors used)

Robert

[Dave]

115,200.... What?

[Charlie]

Actually the main reason for lower value resistors is when there are several devices on the bus. If you are just using one device, 4.7 K is fine and wastes less current.
Robert, you must have posted on the wrong thread? No RS-232 in this discussion...

[Demon]

115,200.... What?

Oops. Overly medicated apparently. LOL