fftpic
I was looking at Table 20-1, Oscillator Operating Modes, in the Microchip dsPIC30F datasheet, but I'm not sure which configuration will give me the best performance. I want to do an FFT and I need the best performance. I use PIC CSS C Compiler.
Which configuration do I need, and what crystal and capacitors should I use?
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.
First: Do you have a 20 MHz crystal or crystal oscillator? Those are two different things. A crystal oscillator will all on its own generate a 20 MHz clock signal for the PIC and you use the external oscillator option with it.
On the other hand, the quartz crystal is an external part of the internal oscillator and internal components together with the crystal and load capacitors make a complete oscillator. In such configuration, you use various crystal modes. Also take a look at figure 2.2 on page 27 of the datasheet.
Now to set up this part correctly, you need to understand a few things, so I'll quote the datasheet:
When the PIC18F4550 is used for USB connectivity, it must have either a 6 MHz or 48 MHz clock for USB operation, depending on whether Low-Speed or Full-Speed mode is being used.
You need to combine things so that the USB clock is 48 MHz or 6 MHz and then you need to set-up the microcontroller operating frequency so that it works at suitable speed. Those two clocks may be different.
On page 26 of the datasheet, you have a nice diagram which you should take time to analyze. The USB PLL input expects 4 MHz frequency which it will use to generate the 96 MHz from which it will derive the operating frequency for USB and the microcontroller.
In your screenshot, the 20 MHz are divided by 5 to get the 4 MHz needed for USB PLL which then raises that to 96 MHz, as seen in the PLL prescaler section.
Then you have the system clock postscaler section. It is currently set to use the 96 MHz created by USB PLL and divided by 2 as the main system clock. You also have other options to set the main systme clock. I can't remember exactly what they are and I've just formatted my HDD, so mikroC isn't installed yet. They should offer you to derive the system clock from an internal oscillator or directly from the clock used to generate the 4 MHz for the USB PLL or as it is shown in the screenshot from the 96 MHz generated by the USB PLL.
The point here is that you can independently select the main clock and the USB clock. For example, if you have a 20 MHz oscillator, you could run the PIC main clock at those 20 MHz and at the same time run the USB clock at needed 48 MHz.
Next you have the oscillator selection part. For real crystal oscillators, you should use EC options and connect the output of the oscillator to the OSC1/CKLI pin (in your case pin 9). You can then use the 20 MHz oscillator to drive the PIC.
In case you're using a crystal, you need to use the crystal options. They are XT, for low frequency crystals, up to 4 MHz, and HS for high frequency crystals up to 20 MHz, if I remember correctly.
As for which crystal is better, well that depends on a lot of things such as which exact crystal you're using, its characteristics, characteristics of the PLL used in the PIC and so on.
Usually low frequency crystals drift less over time and produce cleaner signal while high frequency crystals often give as their output a harmonic of some lower frequency and the signal is usually weaker. I myself would use the 4 MHz crystal here.
Also I forgot the last part of your question: In the "Oscillator frequency" field, you should enter the effective operating frequency of the PIC, that is to say the frequency the "primary clock" on figure 2.1 on page 25 of the datasheet sees. In your particular case, that would be 48 MHz.
So to sum this up: In the 20 MHz crystal case, you should first set the "oscillator selection" to HSPLL. That will give 20 MHz at the input of "primary oscillator" in the above-mentioned figure 2.1. Next, you should set the PLL prescaler to divide by 5, so you get 4 MHz which are multiplied by 24 to get the 96 MHz for USB. Next set the "USB clock selection" to 96 MHz divided by 2 and set the "System clock postscaler selection" to 96 divided by two. Finally, set the Oscillator frequency to 48 MHz and you're done with this part.
For the 4 MHz crystal, you should first set HSPLL. Set the PLL prescaler to divide by 1 and then set the "USB clock selection" to 96 MHz divided by 2 and set the "System clock postscaler selection" to 96 divided by two and set the Oscillator frequency to 48 MHz and that's it.
Best Answer
Typically for programming the more advanced Microchip processors (PIC18/24/32, dsPIC30/33), you need more than the device datasheet to understand the details of operation (for example, how to get the most MIPS).
This information is found in the dsPIC 30F Family Reference Manual in Chapter 7. Specifically, a summary of the modes are found in Section 7.5, and a detailed document regarding choosing the values of the capacitors is found in Section 7.7. Notice that Fcy = Fosc/4, therefore to achieve the maximum device performance of 30 MIPS, you need to have Fosc = 30 * 4 = 120 MHz. One way to do this is to have a crystal in 'XT + 16XPLL' mode at 7.5 MHz, which should give you exactly 30 MIPS (7.5 * 16 / 4). You will quickly find that 7.5 MHz is not a common frequency, since it is not a multiple of the common serial port baud-rates. The closest is usually 7.3728 MHz, which will give you a slight reduction in performance, but let you use the standard baud rates (e.g. it is 115200 * 64).