Regarding I2C interface

Most devices designed to operate on an I2C interface have address pins. Normally the address for I2C devices is made up of 4 class/device type bits (bits 7-4), and 2 or 3 address input bits (bits 3-1 or 2-1, with bit 3 fixed in the case of 2 address bits) and the read/write bit (bit

0) [The r/w bit is required to be bit 0 in the I2C spec].

If you only have two address bits, you are limited to 4 of that device on one bus, unfortunately.

Are the devices you are using as slaves commercially available I2C parts, or custom?

It's hard to know how to help without that information.

Cheers

PeteS

Short answer? You won't. If you've got all 8 chips on the same I2C bus, with the same I2C address..... a) Any attempt to write to any of them will write the same data to all of them. b) Any attempt to read from any of them will return nothing but garbled nonsense.

I'd advise using either a bus switch such as the pca9548a or a bus multiplexer like the pca9544a (you'll need two of them, they're only 4 line muxes) if you really need to have all 8 of them with the same slave address. Actually, I'd really recommend finding a way to design this such that you don't need to have 8 of the same device on the same slave address, but that's just me.

Ummm.....

Read the Philips spec would be a good start.

Btw - you can't update multiple registers * simultaniously * on I2C unless I missed something.

Graham

We take it you mean "from one master".

I second PeteS regarding the address pins and advice to use I2C multiplexers the last bit of the way (or all the way if necessary)...

/A

Hi,

My system looks like this.

-------------- I2C1 | |I2C5 Master1 ----------| |---------- to chip 1 | |I2C6 | |---------- to chip 2 | |I2C7 | |---------- to chip 3 I2C2 | |I2C8 Master2 ----------|Black box |---------- to chip 4 | | | |I2C9 I2C3 | |---------- to chip 5 Master3 ----------| |I2C10 | |---------- to chip 6 | |I2C11 | |---------- to chip 7 I2C4 | |I2C12 Master4 ----------| |---------- to chip 8 | | -------------- The information conveyed from the master is which chip to write to,and also the address of the register along with the configuration information to be written to the selected chip. All the 8 chips work in slave mode only and all have the same I2C address(i.e the reason we are going for 8 I2C buses and all the 8 chips are the same which are video decoder chips need to configure hue , bightness and saturation etc of the video decoder chips , this information is provided from the 4 different master ** Video Fixed point Digital signal processor**).

Now my question is wether the blackbox shown can be implemented using discrete chips(I2Cswitches/I2C mux) OR is it really feasible to implement the black box in an FPGA.

Any solution would be appreciated. Thanks in advance, Praveen

Unless there is something you aren't telling us, you can just hook the chips up using a single two-wire bus; the masters will have to do arbitration, but this is the kind of thing I2C is designed to do.

The I2C slaves are differentiated by setting a different address on their address input lines. For example, the MCP23016 port expander from microchip has 3 inputs that allow one to set the address it'll look for. If I set it to 010 by tying the inputs to the proper voltages, for example, it'll only accept commands on addresses 0100010 (7 bit addressing).

The masters arbitrate by watching the output when the write data. If another master is talking at the same time, one of their output bits is going to be low when they stop driving it low. The winning master doesn't even know it happened. Most hardware already does this. Doing it in software is pretty easy too.

One issue with I2C is that the total capacitance on the bus must be less than 400pF. The pullup resistors for the bus must be chosen so that the bus comes up quickly enough. There are also schemes for increasing the speed, which requires special hardware drivers.

The Phillips site has an I2C specification that describes the whole thing, including fast transfer modes, and 10 bit addressing.

If you are determined to use a black box for some reason you can't tell us, you can do this by with a microcontroller, FPGA, CPLD, etc. Using discrete logic would be a nightmare. You would have to shift the address bytes into a shift register, compare addresses, wait for the slave to be free (since another master might already be using it), take control of the proper bus, etc. However, since slaves can hold off masters indefinitely, you could collect the address, hold off that master, write the address out, and then simply open up a channel between them using an analog switch.

--
Regards,
   Robert Monsen

Hi all,

There is nothing confidential in the black box it is just a I2C hub/switch. The real scenario is that there are four Video Fixed point Digital signal processor(masters) each has a I2C interface.Now these processors will configure the brightness, hue , saturation etc of TVP5150(video decoder),there are 16 such video decoder chips but i had showed only 8 because The slave address select terminal (I2CSEL) enables the use of two TVP5150A decoders tied to the same I2C bus.

I2CSEL WRITE ADDRESS

0 B8h 1 BAh

Now the problem is that any of the processor may configure registers of any video decoder chip. Any suggestions how to go about this design. Wether we can do it using an FPGA(I2Chub) or can we go for any I2C switch.

One more basic doubt i have is wether the I2C interface from the four processors(masters) will be sitting on the same I2C bus, i.e only 2 pins OR 8 pins will come to the black box(FPGA OR I2c switch).

Any solution would be appreciated.

Thanks in advance, Prav

Sounds like you misunderstand how I2C works to be frank.

Why don't you address the I2C chips individually ?

Graham

Use an external multiplexer on the clock line. Tie all the data lines together. A 74hc138 can be addressed with 3 extra lines from the processor. Use the Active Low '*E' input to run the clock to one of 8 I2C devices. Inactive outputs from the 138 are held high (as I recall).

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."

FPGA is severe overkill, this is fairly doable off the shelf.

If only one of the masters is going to want to be writing to the registers at any given time you can just put all four on the same I2C bus, and connect that bus to either a 1-8 or pair of 1-4s as I said earlier.

Gets a little trickier if you want all of them to be able to play ball at the same time. I'd say you can probably get away with four of those pca9548a 1-8 I2C switches that I mentioned earlier. Use each to connect

1 master to all 8 slave buses, each of which has two slaves with different addresses. You'll need to do some back of the envelope number crunching with the data sheet to make sure that the extra capacitance of three switches in the off position won't violate the aforementioned 400pf(?) limit, but it shouldn't.

This does however leave open the question of how to do bus collision detection with the four masters. The I2C master hardware should do it automatically, as Robert said, but caveat emptor on that point. If you can spare 8 I/O lines on each of the masters you can just do an open-drain wired OR where each line represents one slave bus. That way if the I2C bus collision detection doesn't magically fall into place you'll have your own side mechanism where you can set up as simple or as complicated a collision avoidance as necessary. If you can't you could always use an 8 bit GPIO extender on each master bus to implement the same functionality, but that seems to be a bit much.

-------------- I2C1 | |I2C5 Master1 ----------| |---------- to chip 1 | |I2C6 | |---------- to chip 2 | |I2C7 | |---------- to chip 3 I2C2 | |I2C8 Master2 ----------|Black box |---------- to chip 4 | | | |I2C9 I2C3 | |---------- to chip 5 Master3 ----------| |I2C10 | |---------- to chip 6 | |I2C11 | |---------- to chip 7 I2C4 | |I2C12 Master4 ----------| |---------- to chip 8 | | --------------

If you are serious about the simultaneous, your black box needs to be a rather special set of I2C masters. To answer your question, the only sane implementation would be in an FPGA.

If simultaneous is not important, an I2C GPIO extender and a set of CMOS switches would do. Each master would write the "chip number" into the IO extender followed by accessing the respective decoder via its I2C address.

You'll have to make sure that no other master can get in between the transfers.

Kind regards,

Iwo

Ok, I looked up the thing, you only have two addresses to work with.

One hack would be to muck with the addresses... On the left side, with the masters, you have a simple I2C slave, which listens for a set of addresses. Once an address is decoded, you hold off the master using SCL, and use the address input to the video decoder as a chip select. Everybody with a low bit expects the address to be B8. The one guy with a 1 expects the address to be BA. You then output BA as the command on the slave I2C bus as master. Once it's out on the bus, you just start mapping everything one to one...

Then, all of the slaves can be on a single I2C bus, and all of the masters can also be on their own I2C bus. This may be simpler, assuming the I2CSEL pin can be modified on the fly.

--
Regards,
   Robert Monsen

rob, You are telling me to connect all the four masters on to a single I2C bus , but it is not possible in our system, since all the masters have the same address. one more assumption is there will not be any overlap i.e there will not be any case where the two masters are trying to access the same chip.Hence no arbitartion required.

In my understanding i need four I2C slaves in the FPGA which talk to the for masters,and i need 8 I2C masters which talk to 8 I2C slave chips. Some glue logic is required b/w these 4 slaves and 8 masters in FPGA.

Any suggestions would be appreciated.

Thanks in advance, Praveen

Masters don't have addresses, only slaves have. Bus arbitration between masters is done over the slave address + data.

Your master devices may have slave ports, but unless you're actually addressing them, you don't need to worry about it.

I would suggest you read and understand this document:

Yes, this is the overkill Rob talked about. The only reason for doing this kind of stuff is if you need to preload some register settings and then dump them *simultaneously* (= at exactly the same time) into the slaves.

If you truly need to do this, the FPGA is required, but you can optimise a little. You would need a (single) special slave in the FPGA, one which responds to, say 16 I2C addresses. The 4 address bits give you the target information.

The 8 master blocks could be relatively simple, as they don't have to support arbitration.

Then you would need to think about how to synchronise the transfers and read back the acknowledge status.

Kind regards,

Iwo

Hi rob ,

As suggested by u i went through the I2C spec. In the spec , page 17, Fig 16 gives the format for Data transfer from a hardware master-transmitter. which shows that there is a master address.

But u had said that masters do not have any address, only slaves have.

Please clarify

Thanks in advance, Praveen

prav wrote:

Masters don't have addresses, only slaves have. Bus arbitration between masters is done over the slave address + data.

Your master devices may have slave ports, but unless you're actually addressing them, you don't need to worry about it.

I would suggest you read and understand this document:

It's the CBUS protocol, which was designed to live alongside I2C.

It's based on the idea of using an otherwise unallocated I2C slave address to encapsulate a different protocol.

CBUS does have the concept of a master address. In that respect, it is similar to CAN, the masters say who they are and the slaves listen to whatever masters they want.

In your case, I don't think any of your devices speaks CBUS, so the above is probably quite irrelevant.

Kind regards,

Iwo