I'm want to use an external low frequency crystal (32kHz) for the timer in my MSP430G2553 and the internal oscillator at 8MHz, but in the datasheet I can't see how I should use the external crystal for the timer. As there only appears to be one pin(TACLK). So how would the design for the oscillator look?
Electronic – Using external oscillator for MSP430 Timer
msp430oscillator
Related Solutions
Yes, the 16F84 does need an external oscillator. It is a very old PIC.
However almost all of the newer PICs have an Internal RC Oscillator that can be selected, which will be mentioned in the datasheet.
I would really consider getting hold of a newer PIC, something like a 16F690, or 16F1824/16F1828. These are far more current, and can do anything the 16F84 can do and much more.
If you want to use your 16F84 though, either use an external clock (e.g. from 555 timer or oscillator based on e.g. an inverting gate with RC or crystal) or crystal as specified in the datasheet, or if you don't have an external clock or crystal use the RC option.
It doesn't appear to have an internal oscillator capable of generating its own clock, but if you check page 146 of the PIC16F87XA datasheet it shows how you can use an RC oscillator if cost rather than the number of I/O pins is an issue. The PIC Mid-range Family Guide oscillator section includes the following recommendations:
For REXT values below 2.2kΩ , oscillator operation may become unstable, or stop completely. For very high REXT values (e.g. 1 MΩ), the oscillator becomes sensitive to noise, humidity and leakage. Thus, we recommend keeping REXT between 3 kΩ and 100 kΩ.
Although the oscillator will operate with no external capacitor (CEXT = 0 pF), we recommend using values above 20 pF for noise and stability reasons. With no or small external capacitance, the oscillation frequency can vary dramatically due to changes in external capacitances, such as PCB trace capacitance and package lead frame capacitance.
While the design is based on a relaxation oscillator I found several references to Microchip not committing to an exact time determination formula. However on page 200 of the datasheet you'll find a graph of typical frequencies for several resistor / capacitor combinations. If you're running at 5V it looks like 20pF and a 5.1 kΩ resistor should be close to the mark.
I guess you're using a chip you already have on hand, but the more modern devices include internal clock options that are much more accurate and leave the I/O pins available for other uses. If you won't really need that much speed you could also consider running it as a lower clock rate, as 4MHz is the highest recommended rate and component tolerances may push it over that limit.
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Best Answer
To use an external 32.768 KHz crystal with the MSP430G2553, the crystal needs to be soldered between the Xin and Xout pins of the microcontroller. In the DIP package, those are pins 18 and 19. No additional components are required for this.
TA0CLK has nothing to do with this crystal's connection, please see the pin descriptions in the datasheet.
See this schematic of the MSP430 Launchpad Target Microcontroller section:
You can see the two 0 Ohm resistors on the pins 18 and 19 - the crystal attaches between them.
Please note:
UPDATE:
Once the 32.768 KHz watch crystal is connected to the MSP430, the following notes are handy to confirm that the crystal is operational, and of course to use the crystal going forward.
BCSCTL3 |= XCAP_3;for the 12.5 pF watch crystal provided with the Launchpad.