On 15 Apr 2012, at 21:12, David Forbes wrote: > > The DS2321 is not a real part, according to Google. The DS1307 is. > > It's true that you don't need a pullup on SCL, since it's only ever driven by > the CPU. But the SDA line changes direction between writing the pot address > and reading the data. It will drift to an invalid state without a pullup, > which may cause problems unless you do some clever software hacks such as > ensuring that the SDA input is ignored unless it's actively being driven by > the RTC chip. > > Feel free to experiment with I2C with no pullup resistor, but you might want > to use an oscilloscope to show you what happens to the SDA line when it > changes direction.
The part is a DS3231 David. I finally realised that the data line is bidirectional and on a PIC read it will be floating in the breeze so I understand now. I must have been being particularly dense yesterday. I am sure I will find room for some pullups. Grahame thanks for finding the 125mW resistors at Farnell. Somehow when I choose 125mW in the parametric search it only shows up one wired resistor, an out of stock US part at a crazy price, 1885813. When doing small PCB designs it always seems the biggest space hog is clearance for the mounting holes. Thanks to all who helped, John S -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to neonixie-l@googlegroups.com. To unsubscribe from this group, send email to neonixie-l+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
