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MSP430 Ultra-Low Power 16-bit Microcontroller Forum
using an external clock
is it OK to use an external clock instead of a crystal. I have a 1.0Mhz clock avaiiable and would rather just use it instead of a crystal. I am running out of room and would rather not add another part. I am using the MSP430F155
From reading the Users Guide SLAU049F and datasheet SLAS368G you can connect the 1MHz clock signal to XT2IN pin. Note that:
1) The XT2OFF bit in BCSCTL1 should be zero
2) The clock signal needs to meet the Vil and Vih input levels given in the crystal oscillator, XT2 oscillator table in the datasheet.
I agree looking at fig 4.3.2 in the manual confirms. It seems like standard logic to me while anything you do on the XT2 pin gets messy.
Chester Gillon1) The XT2OFF bit in BCSCTL1 should be zero
However, it won't hurt clearing the bit, which is set by default.
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Jens-Michael GrossThis is not really necessary. The users guide states "1: XT2 is off if it is not used for MCLK or SMCLK.". Since the external clock does not need any 'startup time', liek a crystal does, it will be immediately available if MCLK or SMCLK are using XT2. So it is a don't care whether XT2 is 'deactivated' when it is not used at all.However, it won't hurt clearing the bit, which is set by default.
XT2 may be used with external clock signals on the XT2IN pin. When used with an external signal, the external frequency must meet the datasheet parameters for XT2 and XT2OFF must be reset.
Chester GillonIt was the following from section 4.2.3 XT2 Oscillator of Users Guide SLAU049F which suggested that it was necessary to clear XT2OFF:
I guess, this discrepancy only is of interest if you want to save the lase nA of supply current while MCLK and SMCLK are off in LPM.
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