Cirrus Logic AN168 - Manual (Page 27)

AN168
AN168REV2 27
1.6.2.3 +12VBATT/BATTGND Components
1) Cell-phone battery charger circuitry
2) Speaker driver (if powered from +12V source)
Most cellular phones provide two grounds at the
hands-free connector, DGND and AGND. DGND
ties to the phone battery, and should be connected
to BATTGND on the hands-free kit. The cell
phone’s analog interface pins are referenced to the
AGND signal, and this AGND should tie to AGND
on the hands-free kit.
The DGND plane on the hands-free kit should be
connected to AGND plane in exactly one place.
1.7 Quick list of important points:
The following is a list of things to verify in the
system.
1.7.1 Reset and configuration timing
It is vital that the CS6422 receive a cold reset after
power-on.
1) The RST low time should be at least 1
µs.
2) The time between RST rising and DRDY fall-
ing should be at least 110 ms. This is common-
ly overlooked.
3) Furthermore, when writing multiple control
words to the CS6422, the words themselves
cannot be written at a rate faster than one every
125
µs, since this is the rate at which they are
polled by the DSP. That is, the time from
DRDY falling to DRDY falling must be at least
125
µs.
In implementation, it is easiest to ensure this timing
by limiting the STROBE frequency. Setting the
STROBE period to 10
µs, for example, makes the
total time for a register write 200
µs (16 data bits +
4 extra STROBE pulses = 20 cycles; 20 cycles *
10
µs = 200 µs).
1.7.2 Distortion
The total distortion present in any signal path that
the echo cancelers are trying to model should be
limited to 2% THD, worst case across the
frequency band of interest.
1.7.3 Speaker/mic placement
The system should be tested in a car, not a lab. The
microphone should be placed near the driver’s
visor or rear-view mirror. The speaker should be
placed on top of the dash, or in some location such
that there is a minimum of movement in the air
space between the speaker and the microphone.
> 1 µs
> 110 ms
> 125 µs
3
2
1
RST
DRDY
Figure 20. CS6422 Reset and Configuration Timing