Functional Description
3 - 9
TUA6024
Wireless Components
Specification, December 1999
By means of control bit CP the pump current can be switched between two val-
ues by software. This programmability permits alteration of the control response
of the PLL in the locked-in state. In this way different VCO gains can be com-
pensated, for example.
The software-switched ports PLOW, PMID and PHIGH are general-purpose
open-collector outputs. The test bit T1 = 1, switches the test signals fref
(i.e.fXTAL / 64) and fdiv (divided input signal) to PLOW and PMID respectively.
The lock detector resets the lock flag FL if the width of the charge pump current
pulses is wider than the period of the crystal oscillator (i.e. 250 ns). Hence, if FL
= 1, the maximum deviation of the input frequency from the programmed fre-
quency is given by
f = ± I
P (KVCO / fXTAL) (C1+C2) / (C1C2)
where IP is the charge pump current, KVCO the VCO gain, fXTAL the crystal oscil-
). As the charge pump pulses at i.e. 62.5 kHz (= fref), it takes a maximum of 16
s for FL to be reset after the loop has lost lock state.
Once FL has been reset, it is set only if the charge pump pulse width is less than
250 ns for eight consecutive fref periods. Therefore it takes between 128 and
144
s for FL to be set after the loop regains lock.
3.4.3
I2C-Bus Interface
Data is exchanged between the processor and the PLL via the I2C bus. The
clock is generated by the processor (input SCL), while pin SDA functions as an
input or output depending on the direction of the data (open collector, external
pull-up resistor). Both inputs have hysteresis and a low-pass characteristic,
which enhance the noise immunity of the I2C bus.
The data from the processor pass through an I2C bus controller. Depending on
their function the data are subsequently stored in registers. If the bus is free,
both lines will be in the marking state (SDA, SCL are HIGH). Each telegram
begins with the start condition and ends with the stop condition. Start condition:
SDA goes LOW, while SCL remains HIGH. Stop condition: SDA goes HIGH
while SCL remains HIGH. All further information transfer takes place during
SCL = LOW, and the data is forwarded to the control logic on the positive clock
edge.
referred to the following description. All telegrams are transmitted byte-by-byte,
followed by a ninth clock pulse, during which the control logic returns the SDA
line to LOW (acknowledge condition). The first byte is comprised of seven
address bits. These are used by the processor to select the PLL from several
peripheral components (chip select). The LSB bit (R/W) determines whether
data are written into (R/W = 0) or read from (R/W = 1) the PLL.