8.3.13.2 Automatic Ramping
Automatic ramping is enabled when RAMP_EN = 1 and RAMP_MANUAL = 0. The action of programming FCAL = 1 starts the ramping. In this mode, there are two ramps that one can use to set the length and frequency change. In addition to this, there are ramp limits that can be used to create more complicated waveforms.
Automatic ramping can really be divided into two classes depending on if the VCO must calibrate in the middle of the ramping waveform or not. If the VCO can go the entire range without calibrating, this is calibration-free ramping, which is shown in Typical Characteristics. Note that this range is less at hot temperatures and for lower frequency VCOs. This range is not ensured, so margin must be built into the design.
For waveforms that are NOT calibration free, the slew rate of the ramp must be kept less than 250 kHz/µs. Also, for all automatic ramping waveforms, be aware that there is a very small phase disturbance as the VCO crosses over the integer boundary, so one might consider using the input multiplier to avoid these or timing the VCO calibrations at integer boundaries.
Table 16. Automatic Ramping Field Descriptions
FIELD |
PROGRAMMING |
DESCRIPTION |
RAMP_DLY |
0 = One clock cycle
1 = Two clock cycles |
Normally, the ramp clock is equal to the phase detector frequency. When this feature is enabled, it reduces the ramp clock by a factor of 2. |
RAMP0_LEN
RAMP1_LEN |
0 to 65535 |
This is the length of the ramp in clock cycles. Note that the VCO calibration time is added to this time. |
RAMP0_INC
RAMP1_INC |
0 to 230 – 1 |
2’s complement of the value for the ramp increment. |
RAMP0_NEXT
RAMP1_NEXT |
0 = RAMP0
1 = RAMP1 |
Defines which ramp comes after the current ramp. |
RAMP0_NEXT_TRIG
RAMP1_NEXT_TRIG |
0 = Timeout counter
1 = Trigger A
2 = Trigger B
3 = Reserved |
Determines what triggers the action of the next ramp occurrence. |
RAMP_TRIG_A
RAMP_TRIG_B |
0 = Disabled
1 = RampClk rising edge
2 = RampDir rising edge
4 = Always triggered
9 = RampClk falling edge
10 = RampDir falling edge
All other States = invalid |
This field defines the ramp trigger. |
RAMP0_RST
RAMP1_RST |
0 = Disabled
1 = Enabled |
Enabling this bit causes the ramp to reset to the original value when the ramping started. This is useful for roundoff errors. |
RAMP_BURST_COUNT |
0 to 8191 |
This is the number the ramping pattern repeats and only applies for a terminating ramping pattern. |
RAMP_BURST_TRIG |
0 = Ramp Transition
1 = Trigger A
2 = Trigger B
3 = Reserved |
This defines what causes the RAMP_COUNT to increment. |