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Universal voltage-level translation

Hi !

I'm designing an universal JTAG programming/debugging dongle. I've found it a bit problematic to find good logic level shifters for my design. Basically I need output buffers interfacing 3.3V MCU with "device under test" which may operate at 1.8V-5V.

My needs are as follows:

- all buffers are controlled by MCU with 3.3V I/O

- outputs can operate at any voltage between 1.8V and 5V, output Vcc is taken from the "device under test"

- outputs have to be three-state kind. I need 3 signals with HiZ controlled in common, and 2 signals with separate OEs.

Quick research led me to the following:

  1. I can use dual-supply level translation from TI, series SN74LVC1T45, SN74LVC2T45 etc. they have the right voltage ranges, unfrotunately no three-state capabilities (only bigger ones, 8bits wide and wider).

  1. Maxim's dual-supply translators, like MAX3390 - there are many different input/output configurations so I could choose something for me, however there are 2 requirements complicating the thing:

- one of the supply voltages (called VL) always need to be lower or equal compared to another one (VCC) - I can eventually use 5V MCU I/O

- THREESTATE has to be driven from VL side :( I need it to be driven by MCU, so from VCC side.

  1. I've found NC7SZ125 from Fairchild. It's three-state buffer with VCC range 1.65V - 5V, and overvoltage tolerant inputs - quite ok for me. Still I need 5V MCU I/O to drive its inputs, if I want to support full VCC range, but I can live with it (additional 3.3V -> 5V shifter would do the trick). Does anyone know similar parts, with such a nice VCC range and 5V-tolerant inputs ? Maybe more than one buffer per chip ?

Anyone here designed similar thing ? Any suggestions welcome..

BR, Przemek

Reply to
czajnik
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Have you looked at TI's 74LVC2T45, which are dual rail 1.65 to 5.5V bidirectional transceivers. Might be overkill for your application, but the solved all level shifting problems I've encountered so far.

Klaus

Reply to
Klaus Bahner

Yes, I even mentioned this part in my post - problem is that it lacks OE input (HiZ mode).

BR, Przemek

Reply to
czajnik

Have you already seen this note...

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...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

Many commercial programmers meet similar needs with not digital buffers, but with analog parts, e.g. op-amps (sometimes with push-pull pairs on the output for extra current push or to explicitly tri-state the output) and comparators rigged as schmitt triggers, and digitally-programmed power supplies setting the rails. I think some TI JTAG app notes show example circuits.

Do you have any specs for rise/fall time, capacitive load, ?

Tim.

Reply to
Tim Shoppa

I've just found it today, however I've already seen most of TI shifters datasheets :) Still it's a good review of what TI offers.

Best Regards, Przemek

Reply to
Przemyslaw Wegrzyn

I've seen fast comparators used for inputs, already. Yet I've never seen outputs done in analog way. Any schematics,articles,app notes welcome.

As above, any pointers welcome. I'll search TI's site later today.

Not yet, really :)

BR, Przemek

Reply to
Przemyslaw Wegrzyn

Personally, I just roll my own. Unfortunately they're not stand-alone chips, they're part of a much larger ASIC. I think there's a hidden example in one of my USB patents.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

I searched my desk but didn't see what I thought I remembered from TI's app notes.

But if you look at Microchip's TB013 they show nice drivers for delivering arbitrary programming voltages with op-amps and push-pull drivers. A small change lets you tristate the drivers.

Tim.

Reply to
Tim Shoppa

Sorry, I obviously didn't read your posting carefully enough. Anyway, with regard to the tri-state requirement, can't you just switch transceiver direction? Then your target system does see inputs, which is not exactly a Hi-Z state, but if I remember it correctly the input current of the 2T45 is only some microamps, which may be acceptable. Of course this would require that your MCU is not confused by the "random" output signals of the 2T45, but it shouldn't be too hard to disable, tri-state or whaterver the affected MCU port pins.

Klaus

Reply to
Klaus Bahner

I've finally found a little time to review it - the schematic there seems to contain OP-AMP voltage followers with push-pull output stages, i.e. simple current amps.

According to the app note, it's mainly needed due to capacitive loads on VPP/RESET line. In my case I will not need such a high drive capabilities, yet thanks for this pointer.

BR, Przemek

Reply to
Przemyslaw Wegrzyn

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