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I am looking for a small USB to RS232 (or TTL serial) converter. FTDI was the first stop, but the smallest IC they have is in SSOP28 or QFN32. I'm looking to go even smaller. I don't need high speed - max probably 38400 baud, and USB Low Speed or Full Speed class. I can see how the pins are used up: some for data (TX, RX, CTS, RTS), USB pins (D+, D-, 5V), voltage regulator, crystal, etc.

I'm not sure what product category I should be looking for, other than FTDI's website, which I already checked. I suppose I could also use a USB micro (I already have two micros on the board), but this adds size and complexity to an already complex product. Does anyone know of any possible chips? (preferably also low component count, this is not critical but would be nice.)

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Something like this from Microchip?

MCP2200

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    \$\begingroup\$ @Thomas: Hilarious, I think this proves you never read datasheets. The Microchip MCP2200 QFN20 and FTDI FT232R QFN32 are the exact same size, 5 x 5 mm \$\endgroup\$ – Nick T Nov 17 '10 at 19:43
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    \$\begingroup\$ @Nick T and crystals tend to take a lot of space. \$\endgroup\$ – Kellenjb Nov 17 '10 at 19:45
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    \$\begingroup\$ Instead of the MCP2200, you could get the PIC18F13K50 and add Microchip's CDC firmware yourself. I think this is pretty much what the '2200 is. By compiling yourself, you might have more flexibility with I/O assignments and clock selection. \$\endgroup\$ – markrages Nov 17 '10 at 20:14
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    \$\begingroup\$ @Nick T. the 0.65mm pitch of the QFN20 will be easier to assemble than the 0.50mm pitch of the QFN32. Also the QFN20 land pattern can be made with 8/8 design rules, and the QFN32 will need at least 6/6 rules, so the PCB is more expensive. \$\endgroup\$ – markrages Nov 17 '10 at 20:47
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    \$\begingroup\$ @Thomas They're in stock at microchipdirect.com, all four package types. \$\endgroup\$ – markrages Nov 17 '10 at 22:10
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The only way you are going to really save space is to just use a microcontroller already on your board. This can easily be done by using a microcontroller, which I am sure you have at least a few of, and make sure one was USB. Have it handle the USB, no need for an extra chip.

If you are wanting what FTDI does, as well as FTDI does it, with the same thing but a smaller chip, you are going to be disappointed. The only other way to get around this is remove the need by not using a TTL connection.

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  • \$\begingroup\$ +1 on size, the USB socket is going to be bigger than a QFN32 anyway \$\endgroup\$ – Toby Jaffey Nov 17 '10 at 17:40
  • \$\begingroup\$ @Joby With a micro-USB connector it should be pretty close. \$\endgroup\$ – Nick T Nov 17 '10 at 18:27
  • \$\begingroup\$ The micro USB connector is easily the biggest thing on the board, however, it is good to make all other components smaller to accomodate it. \$\endgroup\$ – Thomas O Nov 17 '10 at 19:22
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    \$\begingroup\$ Some processors offer a separate internal USB clock they generate. I did not say you could do it without replacing a processor. \$\endgroup\$ – Kortuk Nov 17 '10 at 19:46
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    \$\begingroup\$ this answer is spot on, either FTDI, or use a processor with USB support. The advantage of the FTDI is also its drivers are really good and easy to access for serial port applications, proved reliability. Using a uP with USB support and software stack will require limit your uP selection, and require drivers and more software effort. The advantage would be one less device! \$\endgroup\$ – smashtastic Nov 18 '10 at 8:38
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The Arduino Uno uses the Atmega8U2 programmed as a USB to serial converter running LUFA. It's a QFN32.

This chip is a general purpose AVR microcontroller with USB hardware, so can be programmed to support many other USB classes (MIDI, keyboard, mouse etc).

Another choice could be the Silabs CP2102 (QFN28).

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  • \$\begingroup\$ I had to highlight your answer to see that you answered 20 seconds after me, no wonder I missed yours. Same idea, different explanation +1 \$\endgroup\$ – Kortuk Nov 17 '10 at 19:34
  • \$\begingroup\$ @Kor: "highlight your answer" what's that? The "answered xx yyy ago" alt-text timestamp? \$\endgroup\$ – Nick T Nov 17 '10 at 19:48
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a small USB to RS232 (or TTL serial) converter... [smaller than a] SSOP28 or QFN32.

You want something smaller than a 5mm x 5mm QFN32 FTDI FT232RQ ? What are you doing that you are so short on space?

Some options:

  • Upgrade one of the processors you're already using to a processor that supports USB. I agree with Kortuk that this is probably the smallest-total-area approach, even if you have to use a slightly larger processor.

  • don't bother doing USB on-board -- use some simpler protocol (perhaps "RS232 at TTL levels") that wires more-or-less directly to the processor already on your board. Then use some cable with built-in translation from USB at one end to your chosen protocol at the other end, perhaps something like the Sparkfun FTDI Cable but with a smaller connector.

  • Use some other dedicated chip in a smaller form factor, such as the FT231XQ in a 4mm x 4mm QFN-20 package or several other chips in a 3mm x 3mm DFN package.

  • Osamu Tamura CDC-232 shows that it is possible to program a 8-pin processor to translate between USB on one side and RS232 on the other side.

There seem to be many projects that connect such an 8-pin ATTiny45 or a ATTiny85 chip to USB:

The ATtiny45 and ATtiny85 used in those projects are available in several different packages, including 8DIP (for easy prototyping) and a 4.0 mm x 4.0 mm QFN/MLF -- is that small enough for you?

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