CC1020-RTR1 datasheet(51/91 Pages) TI

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CC1020

SWRS046

Page 51 of 92

15.2. VCO and PLL Self-Calibration

compensate

for

supply

voltage,

temperature and process variations, the

VCO and PLL must be calibrated. The

calibration is performed automatically and

sets the maximum VCO tuning range and

optimum charge pump current for PLL

stability. After setting up the device at the

operating frequency, the self-calibration

can

initiated

setting

the

CAL_START bit in the CALIBRATE

internally in the chip, and is valid as long

as power is not turned off. If large supply

voltage drops (typically more than 0.25 V)

or temperature variations (typically more

than 40

oC) occur after calibration, a new

calibration should be performed.

The nominal VCO control voltage is set by

the

CAL_ITERATE[2:0]

bits

the

CALIBRATE register.

The CAL_COMPLETE bit in the STATUS

finished.

The

calibration

wait

time

(CAL_WAIT) is programmable and is

proportional to the internal PLL reference

frequency. The highest possible reference

frequency should be used to get the

minimum

calibration

time.

recommended to use CAL_WAIT[1:0] = 11

in order to get the most accurate loop

bandwidth.

Calibration

time [ms]

Reference frequency [MHz]

CAL_WAIT

1.8432

7.3728

9.8304

49 ms

12 ms

10 ms

60 ms

15 ms

11 ms

71 ms

18 ms

13 ms

109 ms

27 ms

20 ms

Table 24. Typical calibration times

The CAL_COMPLETE bit can also be

monitored at the LOCK pin, configured by

LOCK_SELECT[3:0] = 0101, and used as

an interrupt input to the microcontroller.

To check that the PLL is in lock the user

should monitor the LOCK_CONTINUOUS

bit

the

STATUS

The

LOCK_CONTINUOUS bit can also be

monitored at the LOCK pin, configured by

LOCK_SELECT[3:0] = 0010.

There are separate calibration values for

the two frequency registers. However, dual

calibration is possible if all of the below

conditions apply:

•

The two frequencies A and B differ by

less than 1 MHz

•

Reference

frequencies

are

equal

(REF_DIV_A[2:0] = REF_DIV_B[2:0]

in the CLOCK_A/CLOCK_B registers)

•

VCO

currents

are

equal

(VCO_CURRENT_A[3:0]

VCO_CURRENT_B[3:0] in the VCO

The CAL_DUAL bit in the CALIBRATE

controls

dual

separate

calibration.

The

single

calibration

algorithm

(CAL_DUAL=0) using separate calibration

for RX and TX frequency is illustrated in

Figure

29.

The

same

algorithm

applicable

for

dual

calibration

CAL_DUAL=1. Application Note AN023

CC1020 MCU Interfacing, available from

the Chipcon web site, includes example

source code for single calibration.

Chipcon

recommends

that

single

calibration be used for more robust

operation.

There is a small, but finite, possibility that

the PLL self-calibration will fail. The

calibration routine in the source code

should include a loop so that the PLL is re-

calibrated until PLL lock is achieved if the

PLL does not lock the first time. Refer to

CC1020 Errata Note 004.

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