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Universal voltage-level translation
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czajnik@czajsoft.pl
electronics forum beginner


Joined: 20 Jul 2006
Posts: 4

PostPosted: Thu Jul 20, 2006 12:11 pm    Post subject: Universal voltage-level translation Reply with quote

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).

2. 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 Sad I need it to be driven by
MCU, so from VCC side.

3. 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
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Klaus Bahner
electronics forum beginner


Joined: 12 May 2005
Posts: 25

PostPosted: Thu Jul 20, 2006 1:24 pm    Post subject: Re: Universal voltage-level translation Reply with quote

Quote:
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 ?

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
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czajnik@czajsoft.pl
electronics forum beginner


Joined: 20 Jul 2006
Posts: 4

PostPosted: Thu Jul 20, 2006 2:39 pm    Post subject: Re: Universal voltage-level translation Reply with quote

Klaus Bahner wrote:
Quote:
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 ?

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.

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

BR,
Przemek
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Jim Thompson
electronics forum Guru


Joined: 06 May 2005
Posts: 5440

PostPosted: Thu Jul 20, 2006 2:46 pm    Post subject: Re: Universal voltage-level translation Reply with quote

On 20 Jul 2006 07:39:52 -0700, czajnik@czajsoft.pl wrote:

Quote:
bidirectional transceivers

Have you already seen this note...

http://focus.ti.com/lit/an/scea035a/scea035a.pdf

...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.
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Tim Shoppa
electronics forum Guru


Joined: 03 May 2005
Posts: 414

PostPosted: Thu Jul 20, 2006 3:00 pm    Post subject: Re: Universal voltage-level translation Reply with quote

czajnik@czajsoft.pl wrote:
Quote:
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.

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.
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czajnik@czajsoft.pl
electronics forum beginner


Joined: 20 Jul 2006
Posts: 4

PostPosted: Thu Jul 20, 2006 5:31 pm    Post subject: Re: Universal voltage-level translation Reply with quote

Jim Thompson wrote:


Quote:
Have you already seen this note...

http://focus.ti.com/lit/an/scea035a/scea035a.pdf


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

Best Regards,
Przemek
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czajnik@czajsoft.pl
electronics forum beginner


Joined: 20 Jul 2006
Posts: 4

PostPosted: Thu Jul 20, 2006 5:36 pm    Post subject: Re: Universal voltage-level translation Reply with quote

Tim Shoppa wrote:

Quote:
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

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.

Quote:
digitally-programmed power supplies setting the rails. I think some TI
JTAG app notes show example circuits.

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

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

Not yet, really :)

BR,
Przemek
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Jim Thompson
electronics forum Guru


Joined: 06 May 2005
Posts: 5440

PostPosted: Thu Jul 20, 2006 5:36 pm    Post subject: Re: Universal voltage-level translation Reply with quote

On Thu, 20 Jul 2006 19:31:46 +0200, Przemyslaw Wegrzyn
<czajnik@czajsoft.pl> wrote:

Quote:
Jim Thompson wrote:


Have you already seen this note...

http://focus.ti.com/lit/an/scea035a/scea035a.pdf


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

Best Regards,
Przemek

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.
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Tim Shoppa
electronics forum Guru


Joined: 03 May 2005
Posts: 414

PostPosted: Thu Jul 20, 2006 6:01 pm    Post subject: Re: Universal voltage-level translation Reply with quote

Przemyslaw Wegrzyn wrote:
Quote:
Tim Shoppa wrote:

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

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.

digitally-programmed power supplies setting the rails. I think some TI
JTAG app notes show example circuits.

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

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.
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Klaus Bahner
electronics forum beginner


Joined: 12 May 2005
Posts: 25

PostPosted: Fri Jul 21, 2006 8:13 am    Post subject: Re: Universal voltage-level translation Reply with quote

czajnik@czajsoft.pl wrote:
Quote:
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.


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

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
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