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Message
From: Richard Damon<Richard@D...>
Date: Thu Apr 12 01:40:35 CEST 2007
Subject: [oc] recent i2c questions
The way you drive I2C to meet the spec is to use a tri-state pin, but the
data goes to the ENABLE of the gate (enable if want low), and the data is
set to gnd. This effectively makes the pin an open collector signal. This
works for almost all devices. The only problem would be a device that has a
limited number of Enable signals.
Richard
-----Original Message-----
From: cores-bounces@o... [mailto:cores-bounces@o...] On
Behalf Of Matt Ettus
Sent: Wednesday, April 11, 2007 7:08 PM
To: cores@o...
Subject: [oc] recent i2c questions
I just joined the list, but looked over the recent i2c discussion. I
have a related question which I don't believe was answered.
The directions in the I2C documentation say to use a tri-state pin for
SCL and SDA, but I thought that an open-drain was necessary. My
thinking is this:
I2C is not collision-free by design. The intention is that you can
detect when there is a collision if you intend to send a 1 and the bus
looks like a zero because someone else is driving it low.
In the case where there are 2 devices driving the bus, if they both
drive low, there is no problem. If they both "drive" high (really
disconnected since they are open-drain, letting the pull-up do the weak
driving) then there is no problem. If one drives low and one "drives"
high, the one driving high detects a collision, but nobody's outputs are
shorted since you don't actually drive the bus high, you just let the
bus be pulled up.
However, if you use a plain tri-state output, then when you drive high,
you are really driving the bus high, and someone else driving the bus
low will result in a short.
Am I missing something here? The only other option I can see is to
de-assert the OE signal when you want to send a 1, thus turning off the
drive and simulating an open drain, but I don't know if that works in
practice. Most FPGAs don't have real open-drain outputs, do they?
Thanks,
Matt
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