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I'm working on getting an ATMega168 to talk to my GPS that I ripped out of the casing from Microsoft Streets and Trips. I know it talks RS232 @ 5v (according to this post) and I just need to listen on pin 3 and apply 5v to pin 5 and ground to pin 2. I'm having a hard time finding good info on how to use the MAX232 and I just followed a diagram from somewhere but I omitted all capacitors as I need to get some. I'm using the FTDI from an Arduino to forward any TTL to my computer and I get a special "y" symbol coming in when I unplug the data out from the GPS and I get the initial "Hello World" serial message I have in there, so I think the MCU side is all good. Do I really need these caps that I ommitted on the MAX232 chip? Why? Is there some other issue? How do I troubleshoot? I've got no scope.

Here's my pinout for the MAX232:

2 -> +5v
6 -> GND
8 -> GPS RX (pin 3)
9 -> MCU Digital in (software UART)
15 -> GND
16 -> +5v
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The MAX232 is a charge pump. It fills a cap with charge, then moves it to the other side of the supply rail. For example, if you charge a cap on the 5V rail, then disconnect it and connect the low side of the cap to +5V, the high side will now be at +10V.

In short, the capacitors are how the MAX232 makes its high voltages from the lower supply voltage. It will not function without caps.

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  • \$\begingroup\$ In the diagram I'm looking at, they are using 3 1uf electrolytic caps. How do I determine what I can use? Can I use ceramic? 25uf? \$\endgroup\$ – Matt Williamson Nov 30 '10 at 17:16
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    \$\begingroup\$ The max232 datasheet is a free download. \$\endgroup\$ – markrages Nov 30 '10 at 17:31
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    \$\begingroup\$ page 17. datasheets.maxim-ic.com/en/ds/MAX220-MAX249.pdf \$\endgroup\$ – Kortuk Nov 30 '10 at 17:35
  • \$\begingroup\$ I see, so it's a fixed number for capacitance for each particular model... \$\endgroup\$ – Matt Williamson Nov 30 '10 at 18:25
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    \$\begingroup\$ @Matt: The main difference between the whole swath of MAX232-related chips is ones that either operate faster and/or with less capacitance (to allow physically smaller--and cheaper--caps) @mark: I think the MAX232 is actually two charge pumps, a voltage doubler (to get ~+9 V) and voltage inverter (for -9 V) \$\endgroup\$ – Nick T Nov 30 '10 at 19:53
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If the GPS has a TTL async serial port and the AVR has as a TTL async serial port, I don't think you need a (MAX232) level shifter, since neither does the full voltage RS232 (like a PC, modem or a terminal). Are you sure this whole thing is necessary?

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  • \$\begingroup\$ No, I'm not sure. I just read it somewhere. I went along with it because I didn't see anything spit out of the UART, but last night I discovered a short between the serial out and ground on the GPS due to poor soldering of male headers on my part.I'm still working on getting it fixed. \$\endgroup\$ – Matt Williamson Dec 1 '10 at 20:42

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