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[OT] Transmission gates as I2C bus switch

 
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kender



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[OT] Transmission gates as I2C bus switch
PostPosted: Tue Mar 17, 2009 3:30 pm     Reply with quote
Colleagues,

It should be possible to use a pair of transmission gates to disconnect and reconnect a branch of an I2C bus. Obviously, the bus would be switched when it’s idle.

What are your thoughts about this?

- Nick
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FvM



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PostPosted: Tue Mar 17, 2009 3:53 pm     Reply with quote
Should work perfectly. Additional weak pullups must be provided at the disconnected slave side to keep the idle state.
Ttelmah
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PostPosted: Tue Mar 17, 2009 3:55 pm     Reply with quote
Yes.
Downsides:
Some series resistance.
Bus capacitances on the main', and 'selected' parts add up.
You need pull up resistances on all bus segments, and then to be very careful, that when these are connected in parallel (with two sections joined), resistance is still in spec.

Alternative, use a proper I2C multiplexer. Look (for instance), at the Texas PCA9543. This is an I2C device itself, that allows one bus to connect to two others. There is also a 9546, that supports four external busses.

Best Wishes
kender



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PostPosted: Wed Mar 18, 2009 5:13 am     Reply with quote
Thanks, guys! It also looks like the P82B96 can be cleverly used as a bus switch, if it's Vcc is switched: http://www.standardics.nxp.com/literature/presentations/i2c/pdf/hot.plug.p82b96.pdf

BTW, I'm researching the algorithms for recovering the stuck I2C bus. Any tips?
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Ttelmah
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PostPosted: Wed Mar 18, 2009 9:35 am     Reply with quote
Are you controlling the master or the slave?.
Have some kit, that uses I2C, at 12v, with (funnily enough), the P82B96 transceivers. As far as the 'master' is concerned, the bus never gave any problems, and nothing the master does, can directly reset a 'hung' slave.
On the slave, used the CCS I2C_ISR_STATE function, and had perpetual problems wth intermittent hangs. Re-wrote the slave, to test the register bits directly, and from that moment on, have never seen a hang. Also added a test, that the 'transfer counter' (incremented whenever a I2C transfer completes), was incrementing, and added a watchdog timeout, if not, with a complete reset of the I2C code. Did a similar thing at the master.
Since this, operational reliability, has been 100%.

Best Wishes
future_
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PostPosted: Wed Mar 18, 2009 10:00 am     Reply with quote
Ttelmah,

What about hung eeproms? I did not look for it yet but a system with 2 pics and 2 fram hangs up at 1mhz, 400khz does fine. When I have time I'll try again to speed things up.

I think only a power cycle will make hung slave chip work again.
kender



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PostPosted: Thu Mar 19, 2009 2:22 pm     Reply with quote
Quote:
Are you controlling the master or the slave?

I’m writing code for both. There are also simple slave devices without firmware (I/O expanders). The challenge is that my bus is hot-pluggable. If a branch is plugged in (and I have to use a pretty bad connector) in the middle of a transaction it may mess-up the transaction and leave the slave device in a bad state.

Quote:
On the slave, used the CCS I2C_ISR_STATE function, and had perpetual problems with intermittent hangs.

I’ve had problems with i2c_isr_state() too. On top of that CCS had changed a behavior of the i2c_isr_state() somewhere between v4.071 and v4.081 without a proper notice.
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kender



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PostPosted: Sun Mar 22, 2009 1:57 am     Reply with quote
kender wrote:
It looks like the P82B96 can be used as a bus switch, if it's Vcc is switched.

This approach actually worked. I have an I/O expander in the same area, and I'm powering P82B96 from one of I/O expander's pins. Here's a scope screenshot.


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