Reply by rickman May 10, 20162016-05-10
On 5/10/2016 7:22 PM, Allan Herriman wrote:
> On Tue, 10 May 2016 13:29:09 -0400, rickman wrote: > >> On 5/10/2016 6:21 AM, Allan Herriman wrote: >>> On Mon, 09 May 2016 13:35:18 -0500, Tim Wescott wrote: >>> >>>> So, I was happily working away, designing in a 2.5V part to talk to >>>> the I2C bus from a 5V processor, when reality caught up to me. >>>> >>>> 2.5V. 5V. I2C. >>>> >>>> It appears that NXP has some I2C level translators -- does anyone have >>>> any mileage with them? Do they work? Anyone have any parts to either >>>> embrace or avoid? I can certainly see some issues with trying to make >>>> it all reliable, and back when NXP was Phillips I had to coerce some >>>> pretty weird stuff into working correctly, up to and including yanking >>>> one of their I2C to microprocessor bus chips off a board and >>>> bit-banging the I2C. So I'm not ready to fully believe in the NXP >>>> parts. >>> >>> >>> I've used a lot of these in multiple designs. Never had a problem. >>> >>> http://www.ti.com/product/PCA9306 >> >> Very much like the quickswitch devices. I'm pretty sure the quickswitch >> devices I've used have a diode drop from the higher of the references to >> drive the gates, so the low voltage is fixed. I wonder how they set the >> gate voltage in these devices since that seems to be controlled by the >> Vref on the low side. The gate voltage should be a bit higher than the >> Vref to allow the high driver to drive the low side to the full Vdd >> without driving it too high. > > I guess they use a third mosfet, with G shorted to D, to measure the > threshold voltage. The 220k resistor (to the higher rail) determines the > drain current. > The I2C pass fets have their gate volages set to the lower voltage rail + > the threshold voltage.
Makes sense, thanks. -- Rick C
Reply by Allan Herriman May 10, 20162016-05-10
On Tue, 10 May 2016 13:29:09 -0400, rickman wrote:

> On 5/10/2016 6:21 AM, Allan Herriman wrote: >> On Mon, 09 May 2016 13:35:18 -0500, Tim Wescott wrote: >> >>> So, I was happily working away, designing in a 2.5V part to talk to >>> the I2C bus from a 5V processor, when reality caught up to me. >>> >>> 2.5V. 5V. I2C. >>> >>> It appears that NXP has some I2C level translators -- does anyone have >>> any mileage with them? Do they work? Anyone have any parts to either >>> embrace or avoid? I can certainly see some issues with trying to make >>> it all reliable, and back when NXP was Phillips I had to coerce some >>> pretty weird stuff into working correctly, up to and including yanking >>> one of their I2C to microprocessor bus chips off a board and >>> bit-banging the I2C. So I'm not ready to fully believe in the NXP >>> parts. >> >> >> I've used a lot of these in multiple designs. Never had a problem. >> >> http://www.ti.com/product/PCA9306 > > Very much like the quickswitch devices. I'm pretty sure the quickswitch > devices I've used have a diode drop from the higher of the references to > drive the gates, so the low voltage is fixed. I wonder how they set the > gate voltage in these devices since that seems to be controlled by the > Vref on the low side. The gate voltage should be a bit higher than the > Vref to allow the high driver to drive the low side to the full Vdd > without driving it too high.
I guess they use a third mosfet, with G shorted to D, to measure the threshold voltage. The 220k resistor (to the higher rail) determines the drain current. The I2C pass fets have their gate volages set to the lower voltage rail + the threshold voltage. Allan
Reply by rickman May 10, 20162016-05-10
On 5/10/2016 6:21 AM, Allan Herriman wrote:
> On Mon, 09 May 2016 13:35:18 -0500, Tim Wescott wrote: > >> So, I was happily working away, designing in a 2.5V part to talk to the >> I2C bus from a 5V processor, when reality caught up to me. >> >> 2.5V. 5V. I2C. >> >> It appears that NXP has some I2C level translators -- does anyone have >> any mileage with them? Do they work? Anyone have any parts to either >> embrace or avoid? I can certainly see some issues with trying to make >> it all reliable, and back when NXP was Phillips I had to coerce some >> pretty weird stuff into working correctly, up to and including yanking >> one of their I2C to microprocessor bus chips off a board and bit-banging >> the I2C. So I'm not ready to fully believe in the NXP parts. > > > I've used a lot of these in multiple designs. Never had a problem. > > http://www.ti.com/product/PCA9306
Very much like the quickswitch devices. I'm pretty sure the quickswitch devices I've used have a diode drop from the higher of the references to drive the gates, so the low voltage is fixed. I wonder how they set the gate voltage in these devices since that seems to be controlled by the Vref on the low side. The gate voltage should be a bit higher than the Vref to allow the high driver to drive the low side to the full Vdd without driving it too high. -- Rick C
Reply by May 10, 20162016-05-10
On Tuesday, 10 May 2016 11:37:38 UTC+1, Allan Herriman  wrote:
> On Tue, 10 May 2016 10:25:55 +0000, Allan Herriman wrote: > > > On Tue, 10 May 2016 10:21:49 +0000, Allan Herriman wrote: > > > >> On Mon, 09 May 2016 13:35:18 -0500, Tim Wescott wrote: > >> > >>> So, I was happily working away, designing in a 2.5V part to talk to > the > >>> I2C bus from a 5V processor, when reality caught up to me. > >>> > >>> 2.5V. 5V. I2C. > >>> > >>> It appears that NXP has some I2C level translators -- does anyone have > >>> any mileage with them? Do they work? Anyone have any parts to either > >>> embrace or avoid? I can certainly see some issues with trying to make > >>> it all reliable, and back when NXP was Phillips I had to coerce some > >>> pretty weird stuff into working correctly, up to and including yanking > >>> one of their I2C to microprocessor bus chips off a board and > >>> bit-banging the I2C. So I'm not ready to fully believe in the NXP > >>> parts. > >> > >> > >> I've used a lot of these in multiple designs. Never had a problem. > >> > >> http://www.ti.com/product/PCA9306 > > > > Here is the same part at NXP. > > > > <http://www.nxp.com/products/interface-and-connectivity/interface-and- > system-management/isup2-supc-bus-portfolio/isup2-supc-voltage-level- > translators/dual-bidirectional-i2c-bus-and-smbus-voltage-level- > translator:PCA9306> > > > BTW, these are (as other posters have pointed out) just a pair of mosfets > with some biasing circuitry to limit the voltages on one of the ports. > (Hint: the PCA9306 "EN" pin will be regulated to the lower supply voltage > + Vgs(th).) > > This is perfectly fine for low speed I2C with its open drain drivers and > resistive pullups. Some of the faster I2C modes have active pullups and > won't work at full speed through this class of voltage level translator. > > Regards, > Allan
I have also used the mosfet method with no problems at 30kHz. John
Reply by Allan Herriman May 10, 20162016-05-10
On Tue, 10 May 2016 10:25:55 +0000, Allan Herriman wrote:

> On Tue, 10 May 2016 10:21:49 +0000, Allan Herriman wrote: > >> On Mon, 09 May 2016 13:35:18 -0500, Tim Wescott wrote: >> >>> So, I was happily working away, designing in a 2.5V part to talk to
the
>>> I2C bus from a 5V processor, when reality caught up to me. >>> >>> 2.5V. 5V. I2C. >>> >>> It appears that NXP has some I2C level translators -- does anyone have >>> any mileage with them? Do they work? Anyone have any parts to either >>> embrace or avoid? I can certainly see some issues with trying to make >>> it all reliable, and back when NXP was Phillips I had to coerce some >>> pretty weird stuff into working correctly, up to and including yanking >>> one of their I2C to microprocessor bus chips off a board and >>> bit-banging the I2C. So I'm not ready to fully believe in the NXP >>> parts. >> >> >> I've used a lot of these in multiple designs. Never had a problem. >> >> http://www.ti.com/product/PCA9306 > > Here is the same part at NXP. > > <http://www.nxp.com/products/interface-and-connectivity/interface-and-
system-management/isup2-supc-bus-portfolio/isup2-supc-voltage-level- translators/dual-bidirectional-i2c-bus-and-smbus-voltage-level- translator:PCA9306> BTW, these are (as other posters have pointed out) just a pair of mosfets with some biasing circuitry to limit the voltages on one of the ports. (Hint: the PCA9306 "EN" pin will be regulated to the lower supply voltage + Vgs(th).) This is perfectly fine for low speed I2C with its open drain drivers and resistive pullups. Some of the faster I2C modes have active pullups and won't work at full speed through this class of voltage level translator. Regards, Allan
Reply by Allan Herriman May 10, 20162016-05-10
On Tue, 10 May 2016 10:21:49 +0000, Allan Herriman wrote:

> On Mon, 09 May 2016 13:35:18 -0500, Tim Wescott wrote: > >> So, I was happily working away, designing in a 2.5V part to talk to the >> I2C bus from a 5V processor, when reality caught up to me. >> >> 2.5V. 5V. I2C. >> >> It appears that NXP has some I2C level translators -- does anyone have >> any mileage with them? Do they work? Anyone have any parts to either >> embrace or avoid? I can certainly see some issues with trying to make >> it all reliable, and back when NXP was Phillips I had to coerce some >> pretty weird stuff into working correctly, up to and including yanking >> one of their I2C to microprocessor bus chips off a board and >> bit-banging the I2C. So I'm not ready to fully believe in the NXP >> parts. > > > I've used a lot of these in multiple designs. Never had a problem. > > http://www.ti.com/product/PCA9306
Here is the same part at NXP. <http://www.nxp.com/products/interface-and-connectivity/interface-and-system-management/isup2-supc-bus-portfolio/isup2-supc-voltage-level-translators/dual-bidirectional-i2c-bus-and-smbus-voltage-level-translator:PCA9306> Regards, Allan
Reply by Allan Herriman May 10, 20162016-05-10
On Mon, 09 May 2016 13:35:18 -0500, Tim Wescott wrote:

> So, I was happily working away, designing in a 2.5V part to talk to the > I2C bus from a 5V processor, when reality caught up to me. > > 2.5V. 5V. I2C. > > It appears that NXP has some I2C level translators -- does anyone have > any mileage with them? Do they work? Anyone have any parts to either > embrace or avoid? I can certainly see some issues with trying to make > it all reliable, and back when NXP was Phillips I had to coerce some > pretty weird stuff into working correctly, up to and including yanking > one of their I2C to microprocessor bus chips off a board and bit-banging > the I2C. So I'm not ready to fully believe in the NXP parts.
I've used a lot of these in multiple designs. Never had a problem. http://www.ti.com/product/PCA9306 Regards, Allan
Reply by rickman May 10, 20162016-05-10
On 5/9/2016 4:32 PM, martinbosch.imb@gmail.com wrote:
> Am Montag, 9. Mai 2016 22:18:15 UTC+2 schrieb papab...@gmail.com: >> On Monday, May 9, 2016 at 2:40:41 PM UTC-5, Tim Wescott wrote: >>> On Mon, 09 May 2016 12:00:56 -0700, papabear546 wrote: >>> >>>> On Monday, May 9, 2016 at 1:35:24 PM UTC-5, Tim Wescott wrote: >>>>> So, I was happily working away, designing in a 2.5V part to talk to the >>>>> I2C bus from a 5V processor, when reality caught up to me. >>>>> >>>>> 2.5V. 5V. I2C. >>>>> >>>>> It appears that NXP has some I2C level translators -- does anyone have >>>>> any mileage with them? Do they work? Anyone have any parts to either >>>>> embrace or avoid? I can certainly see some issues with trying to make >>>>> it all reliable, and back when NXP was Phillips I had to coerce some >>>>> pretty weird stuff into working correctly, up to and including yanking >>>>> one of their I2C to microprocessor bus chips off a board and >>>>> bit-banging the I2C. So I'm not ready to fully believe in the NXP >>>>> parts. >>>>> >>>>> -- >>>>> >>>>> Tim Wescott Wescott Design Services http://www.wescottdesign.com >>>> >>>> The SCL and SDA lines of I2C are open drain / open collector so as long >>>> as your 5V part has TTL levels (0.8V and lower is a zero, 2V and higher >>>> is a one), all you need is two pull-ups to 2.5V. Make the pull-ups >>>> stiff, 1K - 2.2K. >>> >>> Would work -- except that the processor is 5V CMOS levels, so it wouldn't >>> see 2.5V as high. >>> >>> Hence the need for level translators in the first place. >>> >>> -- >>> >>> Tim Wescott >>> Wescott Design Services >>> http://www.wescottdesign.com >> >> http://www.nxp.com/documents/data_sheet/GTL2002.pdf > > Hi, > > Or see figure 45 of the I2C Bus Specification which shows how to connect devices with different voltage levels using MOSFETs. > > Here is a link to the document: > http://www.cs.unc.edu/Research/stc/FAQs/Interfaces/I2C-BusSpec-V2.1.pdf
That's a quickswitch except the quickswitch is powered from the 5 volt side and they lower the voltage to the gate to limit the voltage that can be driven through to the low voltage side. -- Rick C
Reply by May 9, 20162016-05-09
On Monday, May 9, 2016 at 4:39:08 PM UTC-5, papab...@gmail.com wrote:
> On Monday, May 9, 2016 at 4:16:40 PM UTC-5, Tim Wescott wrote: > > On Mon, 09 May 2016 13:18:11 -0700, papabear546 wrote: > > > > > On Monday, May 9, 2016 at 2:40:41 PM UTC-5, Tim Wescott wrote: > > >> On Mon, 09 May 2016 12:00:56 -0700, papabear546 wrote: > > >> > > >> > On Monday, May 9, 2016 at 1:35:24 PM UTC-5, Tim Wescott wrote: > > >> >> So, I was happily working away, designing in a 2.5V part to talk to > > >> >> the I2C bus from a 5V processor, when reality caught up to me. > > >> >> > > >> >> 2.5V. 5V. I2C. > > >> >> > > >> >> It appears that NXP has some I2C level translators -- does anyone > > >> >> have any mileage with them? Do they work? Anyone have any parts to > > >> >> either embrace or avoid? I can certainly see some issues with > > >> >> trying to make it all reliable, and back when NXP was Phillips I had > > >> >> to coerce some pretty weird stuff into working correctly, up to and > > >> >> including yanking one of their I2C to microprocessor bus chips off a > > >> >> board and bit-banging the I2C. So I'm not ready to fully believe in > > >> >> the NXP parts. > > >> >> > > >> >> -- > > >> >> > > >> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com > > >> > > > >> > The SCL and SDA lines of I2C are open drain / open collector so as > > >> > long as your 5V part has TTL levels (0.8V and lower is a zero, 2V and > > >> > higher is a one), all you need is two pull-ups to 2.5V. Make the > > >> > pull-ups stiff, 1K - 2.2K. > > >> > > >> Would work -- except that the processor is 5V CMOS levels, so it > > >> wouldn't see 2.5V as high. > > >> > > >> Hence the need for level translators in the first place. > > >> > > >> -- > > >> > > >> Tim Wescott Wescott Design Services http://www.wescottdesign.com > > > > > > http://www.nxp.com/documents/data_sheet/GTL2002.pdf > > > > So, have you actually used it? Did it work? > > > > I understand the principle involved -- I want to know if anyone's done > > this and been happy. > > > > -- > > > > Tim Wescott > > Wescott Design Services > > http://www.wescottdesign.com > > Yes sir, I have successfully used the GTL2010PW (same family, 14-bits). TI second-sources these parts as SN74GTL20XX.
Oops, should have typed GTL2010 as 10-bits.
Reply by May 9, 20162016-05-09
On Monday, May 9, 2016 at 4:16:40 PM UTC-5, Tim Wescott wrote:
> On Mon, 09 May 2016 13:18:11 -0700, papabear546 wrote: > > > On Monday, May 9, 2016 at 2:40:41 PM UTC-5, Tim Wescott wrote: > >> On Mon, 09 May 2016 12:00:56 -0700, papabear546 wrote: > >> > >> > On Monday, May 9, 2016 at 1:35:24 PM UTC-5, Tim Wescott wrote: > >> >> So, I was happily working away, designing in a 2.5V part to talk to > >> >> the I2C bus from a 5V processor, when reality caught up to me. > >> >> > >> >> 2.5V. 5V. I2C. > >> >> > >> >> It appears that NXP has some I2C level translators -- does anyone > >> >> have any mileage with them? Do they work? Anyone have any parts to > >> >> either embrace or avoid? I can certainly see some issues with > >> >> trying to make it all reliable, and back when NXP was Phillips I had > >> >> to coerce some pretty weird stuff into working correctly, up to and > >> >> including yanking one of their I2C to microprocessor bus chips off a > >> >> board and bit-banging the I2C. So I'm not ready to fully believe in > >> >> the NXP parts. > >> >> > >> >> -- > >> >> > >> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com > >> > > >> > The SCL and SDA lines of I2C are open drain / open collector so as > >> > long as your 5V part has TTL levels (0.8V and lower is a zero, 2V and > >> > higher is a one), all you need is two pull-ups to 2.5V. Make the > >> > pull-ups stiff, 1K - 2.2K. > >> > >> Would work -- except that the processor is 5V CMOS levels, so it > >> wouldn't see 2.5V as high. > >> > >> Hence the need for level translators in the first place. > >> > >> -- > >> > >> Tim Wescott Wescott Design Services http://www.wescottdesign.com > > > > http://www.nxp.com/documents/data_sheet/GTL2002.pdf > > So, have you actually used it? Did it work? > > I understand the principle involved -- I want to know if anyone's done > this and been happy. > > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.com
Yes sir, I have successfully used the GTL2010PW (same family, 14-bits). TI second-sources these parts as SN74GTL20XX.