BelaSigna 300
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Table 7. NON−CRITICAL SIGNALS
Pin Name Description Routing Guideline
CAP0, CAP1 Internal charge pump − capacitor connection Place 100 nF capacitor close to pins
SDA, SCL I2C port Keep as short as possible
GPIO[3..0] General−purpose I/O Not critical
UART_RX, UART_TX General−purpose UART Not critical
PCM_FRAME, PCM_CLK, PCM_OUT,
PCM_IN
PCM port Keep away from analog input lines
LSAD[4..1] Low−speed A/D converters Not critical
SPI_CLK, SPI_CS, SPI_SERI, SPI_SERO Serial peripheral interface port
Connect to EEPROM
Keep away from analog input lines
Audio Inputs
The audio input traces should be as short as possible. The
input impedance of each audio input pad (e.g., AI0, AI1,
AI2, AI3, AI4) is high (approximately 500 kW); therefore a
10 nF capacitor is sufficient to decouple the DC bias. This
capacitor and the internal resistance form a first−order
analog high pass filter whose cutoff frequency can be
calculated by f
3dB
(Hz) = 1/(R x C x 2π), which results in
~30 Hz for a 10 nF capacitor. This 10 nF capacitor value
applies when the preamplifier is being used, in other words,
when a non−unity gain is applied to the signals. When the
preamplifier is by−passed, the impedance is reduced; hence,
the cut−off frequency of the resulting high−pass filter could
be too high. In such a case, the use of a 30−40 nF serial
capacitor is recommended. In cases where line−level analog
inputs without DC bias are used, the capacitor may be
omitted for transparent bass response.
BelaSigna 300 provides microphone power supply
(VREG) and ground (AGND). Keep audio input traces
strictly away from output traces. A 2.0 V microphone bias
might also be provided by the VDBL power supply.
Digital outputs (RCVR) MUST be kept away from
microphone inputs to avoid cross−coupling.
Audio Outputs
The audio output traces should be as short as possible. The
trace length of RCVR+ and RCVR− should be
approximately the same to provide matched impedances.
Recommendation for Unused Pins
The table below shows the recommendation for each pin
when they are not used.
Table 8. RECOMMENDATIONS FOR UNUSED PADS
WLCSP Ball Index BelaSigna 300 Signal Name Recommended Connection when Not Used
B2 RCVR_HP+ Do not connect
C3 RCVR+ Do not connect
A3 RCVR− Do not connect
B4 RCVR_HP− Do not connect
A11 AI4 Connect to AGND
N/A AI3/LOUT3 Connect to AGND
B12 AI2/LOUT2 Connect to AGND
A13 AI1/LOUT1 Connect to AGND
B14 AI0/LOUT0 Connect to AGND
D14 GPIO[4]/LSAD[4] Do not connect
E13 GPIO[3]/LSAD[3] Do not connect
C13 GPIO[2]/LSAD[2] Do not connect
D12 GPIO[1]/LSAD[1]/UART−RX Do not connect
E11 GPIO[0]/UART−TX Do not connect
E9 EXT_CLK Do not connect
E7 SPI_CLK Do not connect
C7 SPI_SERI Do not connect
