Re: regarding I2C protocols

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AFAIK, that's normal when the bus is idle in the meantime.

The idle bus has all drivers loose and both lines up. When the master ends a
transmission, the last thing is the STOP condition: SCL up, then SDA up.
When the next transmission starts, the first thing is the START condition:
SCL still up, SDA down.


Tauno Voipio
tauno voipio @ iki fi

Re: regarding I2C protocols
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I guess that is the answer then.  The condition should not occur, but if
it does due to a defect in one component, it should not cause a problem
in another component.  

To the OP,

How does this change your design?  I would think an empty frame would be
handled like one that is not addressed to your device, no?  


Rick "rickman" Collins
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Re: regarding I2C protocols
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Well - here's my concerns and thinking:

1) It seems that the preferred method is to have a STOP condition
   (SDA rising when SCL=1) on the same SCL high period as a START
   period (SDA falling when SCL=1).  This would look like this:
   SCL  ___|                         |_____
   SDA  _______|                 |_________

2) As far as I can tell the spec says nothing about SCL changing
   between a STOP and START.  I wouldn't see any advantage to it,
   but I couldn't sense it was illegal.  I would suppose any
   clock toggling before a START should just be ignored until a
   START is detected.

3) I was worried about whether a master could "change its mind"
   after issuing a start if it was suddenly occupied with something
   considered more important.  Fortunately, this doesn't seem to
   be a problem.

4) Most of what I'm planning is a straightforward FSM clocked on
   the negedge of SCL.  The START and STOP logic I'm planning on
   using isn't as straightforward.  This was the part that would
   have been messed up if I had to account for multiple START or
   STOP methods.  I wanted to create a START detected signal, and
   use that to tell the FSM when to start monitoring SDA.

5) I could possibly use a high-speed internal clock.  However -
   the goal is a low-power design, and I was told that just
   toggling the clock tree would create unnecessary power

Re: regarding I2C protocols
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I have not given this a lot of thought, but I believe you can use two
FFs (with resets) to detect the start/stop conditions and maintain a
state of disabled/enabled.  

The start FF is clocked on the falling edge of SDA with SCL on the D
input.  This FF will be set on a start condition.  The stop FF will be
clocked on the rising edge of SDA with SCL on the D input.  This FF will
be set on the stop condition.  The start FF being off will hold the stop
FF in reset.  The stop FF being set will reset the start FF.  So the
sequence will be;

1) both FFs clear
2) on start, the start FF is set and the rest of the circuit is enabled
3) on stop, the stop FF is set which clears the start FF
4) the start FF being cleared also clears the stop FF

The only issue I can see with this design is that the stop FF will
generate a reset pulse determined by the time it takes to reset both FFs
plus the routing.  Some people would object to this saying it may
violate the timing requirements of your logic.  If so, you may want to
use the LUT or the OR array with the FF to add some extra delay.  In
general this should work ok since it is basically self timed logic.  

On the other hand, using a synchronous design should not consume much
power.  Unless you are going for power below 100 uA, a low power CPLD
(like the coolrunner) should be able to run at 1 MHz (fast enough for
most I2C chips at 400 kb/s) with power at that level.  


Rick "rickman" Collins
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Re: regarding I2C protocols

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In actual data transmission it's not allowed to change the data when the
clock is high. The start and stop conditions are distinguished from normal
data bits by breaking the data transfer protocol here.

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Yes - the contention is taken care by the open-drain/collector nature of the
bus. The master seeing its output clamped down by another master has to

There is a similar method to synchronise the clocks: any station feeling
that the things are going too fast is allowed to extend the clock-down time
by clamping it. The sending station has to honour the clock clamp and wait.


Tauno Voipio
tauno voipio @ iki fi

Re: regarding I2C protocols
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I had something a little different, but not far off from your suggestion.
Think masking off one signal with the other.

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I won't go into the proprietary details, but I'm doing this work for an
SoC design that might be battery powered in some applications. My boss
is keen on reducing power consumption during a standby mode.

I also apologize if I don't get into specifics about my planned design
that might explain my problems.  As with many in these NG's, I work at
a large company that considers the product I produce confidential.  If
this works well, I (personally) wouldn't be averse to submitting this
as an open source Verilog block.  However - I'd have to make sure this
is OK with my employer.

Yu-Ping Wang
Berkeley, California

Re: regarding I2C protocols
Opencores and Xilinx have VHDL I2C code that might be of use.
Though of course there is no warranty on its correctness.

Re: regarding I2C protocols

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Hi, I have done this with a Lattice 1016 (64 registers)
The start condition, SCL high and SDA falling is to put the device in
lets call it "address compare mode", if the address (Bit 1-7) matches the
device goes
into "read or write mode" depending on bit0 , otherwise the "I'm not
interested mode",
i.e. the "not address compare mode" and "not read read or write mode"

This is determined at the rising edge of the 9th SCL pulse. So the start
condition is
a mode reset command. Note that controllers like the PCF8584 have slow
and falling signals. Your FPGA will be ways too fast for this, so you will
to register the signals to determine the transitions.

Example: clock speed 1Mhz (actual speed)
Not_SCL2 And SCL1 = transitionevent

Tip: in read/write mode where the 9th SCL pulse is used for ACK generation
you can also use the rising edge to generate a read or write pulse to
with a device. With this you can have a continuous 8-bit data stream into or
the device. Ideal for a graphical LCD-display (and whenever there is a
with a high frequency and a low frequency device)

Carel Harmsen

Re: regarding I2C protocols
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Thanks for the answer.  I actually re-read the spec, and noticed
that a STOP following a START in the same SCL high period is illegal.
I'm going to ignore an illegally applied STOP (i.e. illegal STOP
ignored).  I was also worried about the possibility of repeated STOP/

However - as a follow-up question, would it be possible to see SCL
toggle after a STOP before the next START command "in the real world"?
None of the timing diagrams in the spec seem to address this
possibility; all diagrams show SDA and SCL staying high for the
foreseeable future.  I'd guess that the thing to do is simply put
put the I2C slave in a wait state until a START condition is seen.  I
wouldn't see any reason to toggle SCL between a STOP and the next
START, but I haven't seen any real-world designs.

Again - many thanks for the replies.

Re: regarding I2C protocols
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Well that's what really matters (IHO), "the real world". And in the real
world everything is possible. You can have a lousy implemented
micro-controller software I2C or even if you are debugging the hardware you
can accidentally toggle a line. I know applications where they misuse the
spec and by driving the SCL low between STOP and START for a certain time,
they signal other devices for example busmaster takeover. Everything is

If you are designing this for a real application you have to deal with the
real world and you must handle all the situations you can think off.  It's
very clumsy if  your hardware is  'hanging'.  Master controllers are build
by spec's and slaves by sense.


Re: regarding I2C protocols
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I feel it is a bit futile forbidding illegal events because sooner or later
they will happen either through bugs or malfunction.

Personally I would make my I2C slaves enter the idle state whenever they saw
a stop condition (legal or not). Seems a fair and simple way of entering
resetting slave state machines without sending any bytes, even if the spec
forbids it.

If it happens to be an illegal transition then the I2C spec does not oblige
me to do anything in particular, so I can keep my I2C slave logic simple. If
I had to implement things like roll-back after malformed messages, I might
need large n-byte roll-back buffers.

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