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I have a PL2303HX and I just etched my board with a USB to Serial IC (PL2303HX). The problem is that it requires a 12 MHZ crystal but I don't currently have.

Just a side note, the IC is on the same circuit with the Atmega328. When I plug the USB cable the LED on pin 13 blinks once. (it means it's working.)

I tested the board but it didn't work but the PC showed a notification that an Unknown Device has been plugged. The problem might be from the circuit or from the crystal, so I'm just interested in knowing if an IC doesn't have a built in oscillator, will it work without one ?

Edit: Actually it does include one but it needs an external one also. "PL-2303HXD already has an integrated clock generator. There is no need for the external 12MHz crystal oscillator used in HXA/XA PCB."

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    \$\begingroup\$ Well, the definition of require is that it's really needed. \$\endgroup\$ – Keelan Apr 9 '13 at 18:57
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    \$\begingroup\$ Where's the datasheet? \$\endgroup\$ – Matt Young Apr 9 '13 at 19:14
  • \$\begingroup\$ v-comp.kiev.ua/download/pl2303HX.pdf \$\endgroup\$ – moenad Apr 9 '13 at 19:21
  • \$\begingroup\$ As pointed out by "noidea", you are going to need a 12MHZ crystal. To make things a bit easier--Is your Atmega328 running with an external crystal? If yes, then you can change that crystal to 12MHZ and jump a wire from the PB7 (TOSC2) pin to pin 27 on PL2303HX. I believe you would also have to ground pin 28 on PL203HX. \$\endgroup\$ – Suirnder Apr 9 '13 at 20:10
  • \$\begingroup\$ Neither the single LED blink nor the unknown device message remotely indicate that it is "working". The latter for example is triggered by the mere presence of the pullup resistor on one of the USB data lines. \$\endgroup\$ – Chris Stratton May 9 '13 at 20:09
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I'm confused. I just looked at the PL2303HX datasheet (http://www.adafruit.com/datasheets/PL2303HX.pdf) and I see no sign that a crystal is required, or that it is even possible to use one! AFAICT, the clock generator is internal to the chip.

I'd say that your problem lies elsewhere.

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  • \$\begingroup\$ PL-2303HX Edition (Chip Rev D) The problem is that I have revision A. I bought them 10 for as low as 4$. The Revision A does not include a built in oscillator. They stopped manufacturing or providing updates for the Rev A. \$\endgroup\$ – moenad Apr 9 '13 at 19:18
  • \$\begingroup\$ v-comp.kiev.ua/download/pl2303HX.pdf \$\endgroup\$ – moenad Apr 9 '13 at 19:20
  • \$\begingroup\$ Actually, I was reading it doesn include one but it needs an external crystal. \$\endgroup\$ – moenad Apr 9 '13 at 19:29
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"Oscillator" has multiple meanings. A crystal, by itself is not an oscillator; it requires a couple of capacitors (usually user-supplied) and an inverter (usually MCU-supplied) before it becomes an oscillator.

There are also self-contained crystal oscillators (XO) which include all the pieces and put out a CMOS- or TTL-compatible clock signal. These come in DIL8 or DIL14 metal packages with only the corner pins.

A 555 can act as an oscillator when configured for astable operation.

The datasheet shows a part that requires either a crystal across pins 27 and 28, or an oscillator or clock signal fed to pin 27. You must have one of these in place in order for the chip to operate properly.

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