EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: kian0079 on October 01, 2024, 12:43:31 pm
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Hi all,
I have this android hardware development kit from here:
https://www.variscite.com/product/single-board-computers/var-sd410customboard/ (https://www.variscite.com/product/single-board-computers/var-sd410customboard/)
I am looking at the schematics for this dev board and cannot understand some strange voltages and connections that are appearing on the headset jack. The schematics can be found here:
https://www.variscite.com/wp-content/uploads/2017/12/VAR-SD410CustomBoard-Schematics.pdf (https://www.variscite.com/wp-content/uploads/2017/12/VAR-SD410CustomBoard-Schematics.pdf)
I am looking at the audio headset section of the schematics (see attached) which has the following signals
- CDC_MIC_BIAS2 - Unused for my application, R80 is not connected in the dev kit
- CDC_MIC2_P - Connects to positive terminal of microphone
- CDC_HPH_REF - Headset Ground Sensing
- CDC_HPH_R - Connects to positive terminal of right speaker
- CDC_HS_DET - For detecting if a headset is connected to the stereo jack
- CDC_HPH_L - Connects to positive terminal of left speaker
The signals are connected to a System On Module (SOM) chip (see attached). Here are the things that I do not understand and hope to have a good discussion.
Firstly the CDC_HS_DET pin on the SOM is an analog input. Looking at the stereo jack connection, CDC_HS_DET connects to CDC_HPH_L when no headset is inserted. I measured the voltage between CDC_HS_DET and CDC_HPH_REF to be about 0.2V. When a headset is inserted, CDC_HS_DET disconnects from CDC_HPH_L, and I assume its floating (I checked and there is no connection between pin 5 with the rest of the pins on the stereo jack) I measured the voltage between CDC_HS_DET and CDC_HPH_REF to be about 1.8V!! How can this be possible possible???? Would anyone be able to figure out why?
My second question is regarding the CDC_HPH_REF signal. In the schematics, there is no direct connection between CDC_HPH_REF and GND. However, when I used a multimeter to check, CDC_HPH_REF and GND is shorted together!! CDC_HPH_REF and GND is not connected internally inside the SOM. The only possibility of CDC_HPH_REF having a connection to ground is through the 120R 1.2A ferrite bead and ESD protection diode. So how did these 2 signals have a short circuit on the dev kit?
Hope someone can help to figure this out! Thanks in advance!