Rainbow Electronics DS1680 - Manual (Page 12)

DS1680
12 of 23
PROGRAMMABLE DUTY CYCLE
The current required to take an X or Y measurement is V
AVD
/ R
D
. In the case of R
D
= 250O and
V
AVD
= 5V, the current required is 20mA. The average current is the current during the measurement,
multiplied by the ratio of the time the drivers are on, to the power of total sample time. In order to
minimize the average current, the on-time should be limited to the minimum time required for the tablet
RC delay.
Experimental data suggests that a typical RC time constant is between 4µs and 5
µ
s for a resistive touch
screen. In order to achieve 10-bit resolution, the settling time must be eight time constants. This creates a
requirement of a minimum of 80
µ
s on-time total, 40
µ
s for each X and Y measurement.
To provide both low power and high sample rate, the on-time for the X- and Y- measurement duty cycle is
programmable. Bits 4 and 5 (SP0 and SP1) of the control register (0Bh) select the on-time of four
different frequency ranges. The frequencies given are the maximum frequency for that timing range,
which will not violate the 40
µ
s-per- measurement requirement.
SP1 SP0 FREQUENCY RANGE (MHz) AVERAGE
CURRENT (A)
SAMPLES/SEC NO. OF
CYCLES
0* 0* 2.0 870µ 543 5
0 1 2.8 1.217m 760 7
1 0 4.0 1.739m 1086 10
1 1 5.0 2.261m 1359 13
*This is the default setting
Average current is the current required for the measurement, averaged out over the entire sample. This
average current is only related to the measurement phase when the drivers are on. The average current
will be drawn from the V
CC
supply. There is also current associated with the pen-detection phase, the
ADC, and the control logic.
The number of cycles indicated is the number of on-time state cycles. One state cycle is 16 main clock
cycles. If the frequency range is 2.0MHz, the state frequency is 2MHz/16 = 125kHz. There are 230 state
cycles in one complete sample. The number of cycles can be used to calculate the settling time and the
sample rate.
Example 1:
Frequency Range : 2.0MHz
Input Clock Frequency : 1.8432MHz
tsettle = (1 / 1.8432e6) x 16 x 5 = 43.4
µ
s
Iavg = (10 / 230) x 20mA = 870
µ
A
Sample Rate = 1.8432e6 / (16 x 230) = 501 samples/sec
Example 2:
Frequency Range : 2.8MHz
Input Clock Frequency : 1.8432MHz
tsettle = (1 / 1.8432e6) x 16 x 7 = 60.8
µ
s
Iavg = (14 / 230) / x 20mA = 1.217mA
Sample Rate = 1.8432e6 / (16 x 230) = 501 samples/sec