The PLL provides the low-jitter high-accuracy clock needed for USB operation.
The reference clock to the PLL depends on the device configuration. On devices that contain the optional XT2, the reference clock to the PLL is XT2CLK, regardless if XT1 is available. If the device has only XT1, then the reference is XT1CLK. A four-bit prescale counter controlled by the UPQB bits allows division of the reference to generate the PLL update clock. The UPMB bits control the divider in the feedback path and define the multiplication rate of the PLL (see Equation 20).
Where CLKSEL is the PLL reference clock frequency DIVQ is derived from Table 42-1
DVM represents the value of the UPMB field Table 42-2 lists some common clock input frequencies for CLKSEL, along with the appropriate registry settings for generating the nominal 48-MHz clock required by the USB serial engine. For crystal operation, a 4 MHz or higher crystal is required. For the crystal bypass mode of operation, 1.5 MHz is the lowest external clock input possible for CLKSEL.
If USB operation is used in a bus-powered configuration, disabling the PLL is necessary to pass the USB requirement of not consuming more than 500 μA. The UPLLEN bit enables or disables the PLL. The PFDEN bit must be set to enable the phase and frequency discriminator. Out-of-lock, loss-of-signal, and out-of-range are indicated and flagged in the interrupt flags OOLIFG, LOSIFG, OORIand FG, respectively.
Modifying the Divider Values
Updating the values of UPQB (DIVQ) and UPMB (DIVM) to select the desired PLL frequency must occur simultaneously to avoid spurious frequency artifacts. The values of UPQB and UPMB can be calculated and written to their buffer registers; the final update of UPQB and UPMB occurs when the upper byte of UPLLDIVB (UPQB) is written.
PLL Error Indicators
The PLL can detect three kinds of errors. Out-of-lock (OOL) is indicated if a frequency correction is performed in the same direction (that is, up or down) for four consecutive update periods. Loss-of-signal (LOS) is indicated if a frequency correction is performed in the same direction (that is, up or down) for 16 consecutive update periods. Out-of-range (OOR) is indicated if PLL was unable to lock for more than 32 update periods. OOL, LOS, and OOR trigger their respective interrupt flags (USBOOLIFG, USBLOSIFG, USBOORIFG) if errors occur, and interrupts are generated if enabled by their enable bits (USBOOLIE, USBLOSIE, USBOORIE).
PLL Startup Sequence
To achieve the fastest startup of the PLL, the following sequence is recommended.
- Enable VUSB and V18.
- Wait 2 ms for external capacitors to charge, so that proper VUSB is in place. (During this time, the USB registers and buffers can be initialized.)
- Activate the PLL, using the required divider values.
- Wait 2 ms and check PLL. If it stays locked, it is ready to be used.