Datasheet

AD7879/AD7889
Rev. C | Page 20 of 40
CONVERSION TIMING
Conversion timing or update rate is the rate at which the
AD7879 provides converted values from the ADC so that the
XY positions in the touch screen can be updated. In other
words, the update rate is the timing required to give valid
measurements in the sequencer.
Figure 29 shows conversion timing for a conversion sequence.
F
C
D
T
MEASURE
F
C
D
T
MEASURE
F
C
D
T
MEASURE
F
C
D
F
C
D
T
MEASURE
T
MEASURE
T
MEASURE
X+
× M × M × M × M × M × M
Y+ Z1 Z2
VBAT/AUX
TEMP
07667-046
Figure 29. Conversion Timing Sequence
FCD is required before each touch screen measurement (X+,
Y+, Z1, and Z2). This time is required to allow the screen inputs
to settle before converting. If the sequence does not contain any
screen channel (VBAT, AUX, or TEMP), only one FCD is added
at start of the sequence. At the end of the sequence, there is
always another FCD.
T
MEASURE
is the time required to perform one measurement in
the conversion sequence.
T
MEASURE
= [ACQ (2 μs, 4 μs, 8 μs, 16 μs) + T
CONV
(7.5 μs) + T
SORT
(2 μs)]
where:
ACQ is the acquisition time which is programmable in Control
Register 1. For temperature measurements, ACQ is fixed at 16 μs.
T
CONV
(typical ADC conversion time) is specified at 7.5 μs.
T
SORT
is the time needed to sort the new sample within the
median filter array. The T
SORT
value is approximately 2 μs. If a
median filter is not used (MED =0), the T
SORT
value is 0.
T
MEASURE
_
MIN
= 9.5 μs (ACQ = 2 μs, no median filter)
Conversion time per channel depends on the number of
samples to be converted. The number of samples is pro-
grammed using the following median filter settings:
T
CHANNEL
= T
MEASURE
× MED
T
CHANNEL
_
MIN
=9.5 μs (ACQ = 2 μs, MED = 0)
T
CHANNEL
_
MAX
= 376 μs (ACQ = 16 μs, MED = 16)
Update Rate = [FCD + (T
MEASURE
× MED)] × N + FCD + TMR
where:
N = number of channels to be measured (1 to 6).
MED = median filter setting (1, 4, 8, 16).
TMR = timer setting (0 μs to 9.4 ms).
The total update rate depends on the median filter settings and
the number of channels in the conversion sequence. The timer
setting (TMR) allows the user more flexibility to program the
update rate.
For example, if
ACQ = 4 us
MED = 8
N = 2
FCD = 1.024 ms
TMR = 620 μs
T
MEASURE
= 4 + 7.5 + 2 = 13.5 μs
T
CHANNEL
= (13.5 × 8) = 108 μs
Then
Update rate = [1024 + 108] × 2 + 1024 + 620 = 3.9 ms