7. Digitally-Controlled Oscillator (DCO) Mode
The DSPLLs support a DCO mode where their output frequencies are adjustable in pre-defined steps given by frequency step words
(FSTEPW). The frequency adjustments are controlled through the serial interface or by pin control using frequency increments (FINC)
or decrements (FDEC). A FINC will add the frequency step word to the DSPLL output frequency, while a FDEC will decrement it. The
DCO mode is available when the DSPLL is operating in locked mode. Note that the maximum FINC/FDEC update rate, by either hard-
ware or software, is 1 MHz. Each DSPLL being used in DCO mode should have fractional M division enabled by setting the appropriate
M_FRAC_EN_PLLx = 0x3B for proper operation.
Note:
DCO mode is not available when in free run or when in holdover. A large freq step can assert LOL on the relevant DSPLL. The
step sizes and frequency of operation need to be considered with the LOL settings and BW.
Table 7.1. Fractional M Divider Enable Controls
Setting Name
Hex Address [Bit Field]
Function
Si5397
Si5396
M_FRAC_EN_PLLA
0x0421[5:0]
0x0421[5:0]
DSPLL feedback M divider fractional enable.
0x2B: Integer-only division
0x3B Fractional (or Integer) division
Required for DCO operation.
M_FRAC_EN_PLLB
0x0521[5:0]
0x0521[5:0]
M_FRAC_EN_PLLC
0x0621[5:0]
—
M_FRAC_EN_PLLD
0x0721[5:0]
—
Si5397/96 Reference Manual
Digitally-Controlled Oscillator (DCO) Mode
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