Datasheet

Stellaris® LM3S1968 Evaluation Board
January 6, 2010 15
Other Peripherals
Speaker
The LM3S1968 evaluation board's speaker circuit can be used in either tone or waveform mode.
The quick-start application uses tone mode.
In tone mode, the LM3S1968 microcontroller's PWM module directly generates tones within the
audible frequency range. The width of the pulses determines the volume. If only one PWM signal
(PWM2 or PWM3) is used, the non-PWM signal should be configured as a general-purpose
output. For increased speaker volume, PWM2 and PWM3 can be configured as complementary
drive signals. In tone mode, be careful to avoid large DC currents in the speaker.
Waveform mode uses two high-frequency PWM signals to drive a MOSFET H-bridge with an
output filter. This circuit is essentially a Class-D amplifier. The symmetrical 2nd order low-pass L-C
filter has a cut-off frequency of approximately 33 kHz. The microcontroller's PWM module should
be configured with a PWM frequency of at least 100 kHz. Using 500 kHz improves audio quality
even further. Once configured, audio waveform data can be used to update the PWM duty cycle at
a rate equal to the audio sampling rate.
The speaker on the evaluation board has standard 8
Ω impedance. Audio quality can be enhanced
by adding a small, vented enclosure around the speaker.
Push Switches
The EVB has five general-purpose input switches. Four are arranged in a navigation-style
configuration. The fifth functions as a Select switch on PG7. The Select switch also connects to the
WAKE
signal of the Hibernate module which has an internal pull-up resistor. A diode (D2) blocks
current into the PG7 pin when in the Hibernate state.
User LED
A user LED (LED3) is provided for general use. The LED is connected to PG2/PWM0, allowing the
option of either GPIO or PWM control (brightness control). Refer to the Quickstart Application
source code for an example of PWM control.
Bypassing Peripherals
The EVB's on-board peripheral circuits require 15 GPIO lines. This leaves 31 GPIO lines and 8
ADC channels immediately available for connection to external circuits. If an application requires
more GPIO lines, the on-board hardware can be disconnected. The EVB is populated with 15
jumper links, which can be cut with a knife to isolate on-board hardware. The process can be
reversed by installing 0603- 0-ohm chip resistors. Table 2-2 shows the microcontroller
assignments and how to isolate specific pins.
Important: The quickstart application will not run if one or more jumpers are removed.
Table 2-2. Isolating On-Board Hardware
Microcontroller Pin Microcontroller Assignment To Isolate, Remove...
Pin 16 PG3 Up switch JP1
Pin 17 PG2/PWM0 User LED JP2
Pin 26 PA0/U0RX Virtual COM port receive JP4