user66749: I have recently bought an arduino uno R3 (the type where the atmega328 is surface mount and not removable on a mainly blue PCB); the board has two crystals on it one for the clock for the USB serial and the other for the atmega328 clock. Suppose I have built a fully completed circuit using the arduino (this means the arduino needs not to be further connected to a PC).  Will my arduino uno based circuit still work if I  simply replace (desolder) the 16Mhz (for the atmega328)  crystal with a 12Mhz crystal (soldered in its place)? The circuit doesn't use the serial ports or the the crystal for USB serial. Actually the circuit is simply the blink LED example built into the IDE.  I am guessing it will still work but blink slower-but I don't know the answer for sure.

user-1: Sure the device will work, but all time related peripherals won't behave. The USART will run at the wrong baud rate, the millisecond timer's intervals will be too long, and so on.

66749: <p>I have recently bought an arduino uno R3 (the type where the atmega328 is surface mount and not removable on a mainly blue PCB); the board has two crystals on it one for the clock for the USB serial and the other for the atmega328 clock. Suppose I have built a fully completed circuit using the arduino (this means the arduino needs not to be further connected to a PC).  Will my arduino uno based circuit still work if I  simply replace (desolder) the 16Mhz (for the atmega328)  crystal with a 12Mhz crystal (soldered in its place)? The circuit doesn't use the serial ports or the the crystal for USB serial. Actually the circuit is simply the blink LED example built into the IDE.  I am guessing it will still work but blink slower-but I don't know the answer for sure.</p>
 -1: <p>Sure the device will work, but all time related peripherals won't behave. The USART will run at the wrong baud rate, the millisecond timer's intervals will be too long, and so on.</p>
