On 4/15/12 12:33 PM, Quixotic Nixotic wrote:
On 15 Apr 2012, at 18:47, David Forbes wrote:
The generic ports are TTL, but when they get configured for I2C, they become
open-collector. I2C requires the pull-up resistors to implement its tricky
protocol over two wires.
You'll have to find the room for them. If it helps, 1/8 watt resistors are a
bit smaller than 1/4 watt resistors.
I am not using pins that can be configured as an I2C bus per se, but using two
ordinary TTL level pins. Is the issue that there is some intermediate state
between high and low that will send the whole mess haywire if I don't have a
default condition set via resistors?
If only Farnell in the UK sold a resistor with wires that are 1/8w, but I
cannot find any.
If Farnell deliver my Raspberry Pi tomorrow as promised I will be a happy
bunny. I seem to have been allocated one of the first 700 - Farnell got 700 and
RS got 700 last week. That means the Pi Foundation have held back 600 of the
initial 2,000 that came to the UK. Presumably these are being sent out to
developers who are doing all the educational software development in readiness
for the start of the new school year in September.
John S
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.
Both Digikey and Mouser in the USA sell 1/8 watt thru-hole resistors.
Too bad for you.
--
David Forbes, Tucson AZ
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