- A PLL VCO frequency must be set
to > 1500 MHz
- For best PLL performance,
maximize VCO frequency where possible.
- A PLL must always operate in
Fractional Mode (DAC_EN and DSM_EN set to ‘1’). This is true even when the
FB_DIV is a integer.
- A PLL Reference clock frequency
must be > 10 MHz.
- REF_DIV[5:0] must be set to
“000001”.
- Calibration procedures for FRACF PLLs in integer mode or fractional
mode:
-
Programming FRACF in Integer Mode with
calibration (Static or Auto Calibration)
- Set all the common PLL settings (PREDIV, FBDIV,
…)
- DACEN: 0
- DSMEN = 0 (INT MODE)
- FASTCAL=1
- CALBYPASS=0
- CALCNT[2:0]=2
- CALIN[11:0]=0
- CAL_EN=1
- SET PLL_EN = 1
- WAIT FOR PLL_LOCK AND CAL_LOCK GO HI
- CAPTURE AND STORE CALCODE WHEN CAL_LOCK GOES HI
IF IT WILL BE USED LATER
- IF CAL_LOCK times out ( > 2 ms ) then continue
- CAL_EN=0 (OPTIONAL, THIS WILL FREEZE CALIBRATION
& INTERNAL CALCODE)
-
Programming FRACF in Fractional Mode (with
Static Calcode)
- Set all common PLL settings to nearest integer
(PREDIV, FBDIV,...)
- Run calibration as outlined in integer mode
- CAPTURE AND STORE CALCODE WHEN CAL_LOCK GOES HI
IF IT WILL BE USED LATER
- IF CAL_LOCK times out ( > 2 ms ) then
continue
- CAL_EN=0 (THIS WILL FREEZE CALIBRATION &
INTERNAL CALCODE)
- IF CAL_LOCK timed out, keep CALBYPASS=0
- IF CAL_LOCK IS ASSERTED AND CALCODE IS CAPTURED
APPLY CALCODE TO CALIN
- CALBYPASS=1 CALCODE IS SET BY CALIN
- ENABLE FRACTIONAL MODE
-
Programming FRACF in Fractional Mode
(Auto-Calibration Not Allowed)
- Set all the common PLL settings (PREDIV, FBDIV,
…)
- DACEN=1
- DSMEN=1 (FRAC MODE)
- FASTCAL =X
- CALBYPASS=1 CALCODE IS SET BY CALIN
- CALCNT[2:0]=XX
- CALIN[11:0] = 0 (OR DESIRED CALCODE)
- CAL_EN = 0
- SET PLL_EN = 1
- WAIT FOR PLL_LOCK
PLL Integer mode with Calibration enabled is recommended for applications sensitive to
phase noise or jitter: Examples include Refclk for SerDes, PHY, DDR etc. See individual
product datasheets for specific SoC requirements. Calibration can be bypassed for digital
clocks like ARM. If Calibration is bypassed, CALIN needs to be set to 0.
When Calibration is enabled, PLL Lock time will be longer since SOC needs to wait for
CAL_LOCK. Applications sensitive to phase noise must wait for CAL_LOCK to go high.