BitsyXb - User Manual
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110118-0001A
Stereo Headphones
When wiring for stereo headphones, wire blocking
capacitors in series with the BitsyXb SPKR- signals
as shown at right. These capacitors block the DC
component of the audio signal and complete the
conversion from differential to single-ended output
drive. Leave the HP_IN signal pulled high to enable
headphone output.
Select blocking capacitor size based on the lowest frequency your application will need to play
out. Larger capacitors give improved bass response (lower frequency cutoff), but are physically
larger and cost more. The corner frequency for the low-pass filter created by the capacitor and the
headphone speaker is calculated as fo=1/(2
π
R
L
C). A 330 uF capacitor into a 32 ohm headphone
speaker will give a low cutoff frequency of 15 Hz. Use electrolytic capacitors rated for at least
6.3 V.
The pull-down resistors shown in the diagram drain any charge that builds up on the headphone
outputs when headphones are not connected. Use 1 k
Ω
resistors.
Using Stereo Headphones and Speakers in the Same System
Some applications use both headphones and
speakers. You can wire the headphone jack to
automatically switch the amplifier to single-ended
mode when a headphone plug is inserted in the jack.
This will disable the drive to any speakers that are
wired into the system.
Most headphone jacks include mechanical switches
that indicate when a headphone plug has been
inserted. The diagram at right shows a circuit that pulls down the HP_IN signal when a
headphone plug is removed.
For this circuit to work reliably in differential mode, the HP_IN signal must remain below V
HP_IN
through the largest output voltage swings of SPKR_L. Use of 1 k
Ω
resistors meets this
requirement.
4.5
Buses and Data Communications
The BitsyXb has several built-in channels for communication with peripheral and peer devices.
These include EIA-232 and logic-level serial, USB host and client ports, SPI bus and I
2
C.
4.5.1
Serial Ports
The BitsyXb has three processor serial ports configured as follows:
Port
# signals
Connector
Standard
Production options
Serial 1
9-wire
J10
EIA-232
3.3 V logic level
Serial 2
3-wire
J3
3.3 V logic level
Serial 3
5-wire
J3
EIA-232
3.3 V logic level
The standard serial ports (Serial 2 and 3) supply two or four signals: Serial 2 uses TX and RX
("three-wire serial", counting GND), while Serial 3 adds RTS and CTS. Serial 1 uses the
processor "full-featured serial port," which adds four more signals (DTR, DSR, DCD and RI) to
supply the full complement of modem control signals.
