The horn driver is designed to drive two types of piezo horns: three-terminal self-resonant piezos and two-terminal piezos. The HORN_SEL bit configures the horn driver for the three-terminal or two-terminal operation. During operation, 120-kΩ pulldown resistors discharge any residual charge on the piezo element. Because VBST powers the horn driver, the loudness of the horn can be adjusted by changing the VBST voltage with the VBST register bits.