Auto-Zero Function; Pen-Down Detection; Pen-Up Detection (Method 2 – Detect Rising Edge); Temperature Compensation - Motorola DragonBall MC9328MX1 Reference Manual

15.4.3 Auto-Zero Function

Due to the large amount of flicker noise in MOS devices that degrade ADC accuracy, an auto-zero
measurement method is introduced to store the flicker noise data during each set of measurements. This
data will be subtracted from the output sample, by the user's software. It is recommended that the
auto-zero measurement always be enabled.
There are 2 options for the auto-zero point.
• AZ_SEL = 1: this option includes the 200k resistors
• AZ_SEL = 0: this option excludes the 200k resistors

15.4.4 Pen-Down Detection

The pen-down interrupt function is provided for idle mode configuration. In idle mode, PX1 is used as a
trigger pin for pen-down interrupt generation. PX1 is normally pulled-up to VDD by Rpd, while PY2 is
pulled-down to GND by SW6. When the pen is down, PX1 shorts to PY1 and then pulled to GND. A
falling edge is produced which triggers the interrupt circuit to generate a pen-down interrupt.
15.4.5 Pen-Up Detection (Method 1 – Compare Value)
Using this method, a pen up condition is detected by sample value comparison.
Because the sample value of pen up is always smaller than that of pen down; this property can be used to
detect pen up.
The value in the Compare Control Register is compared to every pen sample. If the sample is smaller than
the register value, an interrupt is generated. The user should set a compare value which lies between the
pen-up and minimum pen-down value. The compare value is panel-dependent and the user should
experiment to determine the optimum setting.
15.4.6 Pen-Up Detection (Method 2 – Detect Rising Edge)
Using this method, a pen up condition is detected by sampling the level on PX1 pin, its trigger determined
by level going high. The method is similar to the one used for pen down detection.
This method is available only in Auto XYU or Auto ZXYU mode. During the time slot for U, the switches
are automatically configured to be the same as for pen down detection (idle mode), for example only SW6
is on. PX1 is normally pulled-down by SW6 when the pen touched the screen. When the pen is up, PX1 is
released and pulled up by the external resistor Rpd. When the level goes high it is reflected by PUIS bit in
the interrupt status register.

15.4.7 Temperature Compensation

In general, the characteristics of the ADC changes with temperature. To keep the ADC stable, either a
compensation circuit or a compensation algorithm is required. Because a temperature compensation circuit
does not exist in the ASP module, software must be used for compensation.
Auto calibration mode provides the necessary switch settings to help software provide temperature
compensation.
MOTOROLA Analog Signal Processor (ASP)
Pen ADC (PADC) Operation
15-7