With the continuous development of smart headphones, they are divided into three segments (without MIC) and four segments (with MIC) according to their structure. The difference between three segment headphones and four segment headphones is that the three segment MIC and GND are connected together. 1. Common earphone structure: Three segment (without mic) earphones have three contacts, starting from the top segment, which are left channel, right channel, and common ground. The earphone plug is divided into two types: 3.5mm and 2.5mm. Currently, most earphone interfaces use 3.5mm, but the two are not universal. A 3.5mm earphone needs a connector when used on 2.5mm earphone devices; Four segment (with mic) earphones have four contacts, which separate the grounded section into two parts. According to the location of the mic on the connector, it can be divided into European and American standards. The European standard, also known as the national standard (OMTP standard), is used by most models of Huawei and Xiaomi; The American standard, also known as the CTIA standard, is typically the iPhone. The four segment European standard starts from the top segment, followed by left channel, right channel, common ground, Mic (digital signal), while the American standard starts from the top segment, followed by left channel, right channel, Mic (digital signal), and common ground.

Without standards, the earphones detect foot L_ DET, when the headphones are not plugged in, is connected to the VDD and is of high voltage. When the headphones are plugged in, L_ DET will communicate with HP_ L contact becomes low voltage. When the circuit detects L_ When the DET signal changes from 1 to 0, the system assumes that the headphones have been inserted; When the headphones are unplugged, the port signal changes from 0 to 1, and the system assumes that the headphones have been inserted. Based on the changing characteristics of this signal, it can be determined whether to insert or remove the headphones.

From the structural diagram, it can be seen that the difference between a three segment earphone and a four segment earphone is that the MIC and GND of the three segment earphone are connected together. Therefore, as long as MIC=0, it is a three segment earphone. If there is voltage on the MIC (based on MIC impedance, the voltage of the MicDET pin is generally above 0.9V), it is considered a four segment earphone.