Datasheet

Divider
1/2/4/8
DV
SS
DV
CC
TAxCLK
1
0
1
TASSELx
3
2
1
0
ID.x
16-bit Timer
TAR
Capture Register
CCRx
Part of Digital I/OPx.y
Part of Timer_A
PAD
Digital I/O Operation www.ti.com
When a port pin is selected as an input to a peripheral, the input signal to the peripheral is a latched
representation of the signal at the device pin. While PxSELx = 1, the internal input signal follows the signal
at the pin. However, if the PxSELx = 0, the input to the peripheral maintains the value of the input signal at
the device pin before the PxSELx bit was reset.
8.2.6 Pin Oscillator
Some MSP430 devices have a pin oscillator function built-in to some pins. The pin oscillator function may
be used in capacitive touch sensing applications to eliminate external passive components. Additionally,
the pin oscillator may be used in sensor applications.
No external components to create the oscillation
Capacitive sensors can be connected directly to MSP430 pin
Robust, typical built-in hysteresis of ~0.7 V
When the pin oscillator function is enabled, other pin configurations are overwritten. The output driver is
turned off while the weak pullup/pulldown is enabled and controlled by the voltage level on the pin itself.
The voltage on the I/O is fed into the Schmitt trigger of the pin and then routed to a timer. The connection
to the timer is device specific and, thus, defined in the device-specific data sheet. The Schmitt-trigger
output is inverted and then decides if the pullup or the pulldown is enabled. Due to the inversion, the pin
starts to oscillate as soon as the pin oscillator pin configuration is selected. Some of the pin-oscillator
outputs are combined by a logical OR before routing to a timer clock input or timer capture channel.
Therefore, only one pin oscillator should be enabled at a time. The oscillation frequency of each pin is
defined by the load on the pin and by the I/O type. I/Os with analog functions typically show a lower
oscillation frequency than pure digital I/Os. See the device-specific data sheet for details. Pins without
external load show typical oscillation frequencies of 1 MHz to 3 MHz.
Pin oscillator in a cap touch application
A typical touch pad application using the pin oscilator is shown in Figure 8-1.
Figure 8-1. Example Circuitry and Configuration using the Pin Oscillator
A change of the capacitance of the touch pad (external capacitive load) has an effect on the pin oscillator
frequency. An approaching finger tip increases the capacitance of the touch pad thus leads to a lower self-
oscillation frequency due to the longer charging time. The oscillation frequency can directly be captured in
a built-in Timer channel. The typical sensitivity of a pin is shown in Figure 8-2.
330
Digital I/O SLAU144J–December 2004–Revised July 2013
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