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On a microcontroller board that I am working on, there is a buffer chip between the microcontroller TI msp430 F1611 and USB interface FTDI FT232BL. The chip is named NC7WZ126.

My question is, why they have used that buffer? what benefits it has and what would happen if that chip was not there at all?

I am asking this because I want to wire up directly to TX/RX pins of the msp430 for some debugging purpose. So should I directly wire to the msp430 pins (Pins #34 and #35)? Or I should wire to the inputs of the buffer (pin 2 and 3 of NC7WZ126)

UPDATE

AFAIK msp430 is using 0-3.3V for low-high (Since its get its Vcc after a 3.3 regulator) bits and that FTDI is 5 Volts (powered by USB) so can be that a reason for that buffer? Considering That I want to feed in 5V CMOS to RX/TX of msp 430, then I should wire to the buffer and not to the 430?

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Yes, the buffer is performing logic level translation. I know I've answered this exact question before, but I doubt that I can find that other one now. – Dave Tweed Dec 22 '12 at 1:34
possible duplicate of 74LVC1G125 on the arduino wireless shield? – Dave Tweed Dec 22 '12 at 1:39
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Actually, it's the exact same answer to a different question. There's no way you would have found this answer based on what you were looking for. – Dave Tweed Dec 22 '12 at 1:40
@DaveTweed Thanks that also applies! – Sean87 Dec 22 '12 at 1:48
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I agree that the purpose of that chip is probably to do level shifting, but that is a bit strange, because the FT232 can easily operate its uC-side pins at 3v3. – Wouter van Ooijen Dec 22 '12 at 7:58

2 Answers

up vote 2 down vote accepted

Without looking at the details, you should be able to figure that out once you read this and then go get the datasheets.

THere are three reasons to buffer:

1) to do the voltage translation from one domain to another. - is the msp430 running at a different voltage that what the USB is specified for?

2) The driver provides higher current drive for a higher load. - often the pins are limited drive (current capability)

3) to protect the more expensive part which might be more sensitive to ESD etc. - added for completeness

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Thanks for quick answer...I updated my question, can you check please? – Sean87 Dec 22 '12 at 1:31
I would say that was the reason ... – rawbrawb Dec 22 '12 at 1:33
up vote 1 down vote

The board you're working on is likely similar to Telos B sensor nodes that were floating around a couple years back. The Circuitry formed around the FTDI chip and including the NC7WZ126 is used to create a specific bit pattern on the lines used to invoke the Bootstrap loader of the MSP430 so that the binary image can be downloaded.

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Yes exactly its TelosB. So can you tell me please if I want to wire directly to UART1RX pin of MSP430 in telosb, I should forget about that buffer and wire directly to MSP430 pins? – Sean87 Dec 22 '12 at 2:47
Haha at first I though you are a magician but I recalled that you answered my last question that I mentioned TelosB in it! Or maybe you are a magician! Please tell me how did you figure that out so I can stop freaking out! – Sean87 Dec 22 '12 at 2:49
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There are not that many FTDI designs with that particualr chip. I used to design wireless sensor nodes and used a similar configuration to access TI's BSL. – Gustavo Litovsky Dec 22 '12 at 2:52
Then I think you are the right person to answer! So might you give your opinion of where should I solder my TX/RX wires to UART1 of msp430 to avoid the disappointment of UART0 in TinyOS? Since UART0 is shared with radio and flash memory on TelosB its next to impossible to use it as a trusty interface, even with resource arbitration its not doing good. I want to use UART1 instead which has no limits AFAIK. Thanks! – Sean87 Dec 22 '12 at 2:59

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