ssHeadUnit

An Android application that turns a tablet into an Android Auto head unit — the same role a factory car head unit plays. Plug a phone into the tablet with a USB cable and the phone starts Android Auto and projects its interface onto the tablet, with touch input sent back to the phone.

How it works A car head unit is the USB host and the phone is the accessory:

AOAP – the tablet asks the phone to switch into Android Open Accessory mode by sending the accessory identification strings a head unit sends (Android / Android Auto / version 2.0.1). The phone re-enumerates with a Google vendor id and exposes two bulk endpoints. Version negotiation – the phone sends a version request on the control channel and the head unit answers with its protocol version. TLS handshake – the head unit is the TLS server; the handshake records are tunnelled inside SSL_HANDSHAKE control messages. Every later frame payload is encrypted. Service discovery – the head unit advertises its channels: video, media/speech/system audio, touch input and sensors (driving status and night mode). Channel setup – the phone opens channels, negotiates the A/V configuration and then streams H.264 video and PCM audio. The head unit acknowledges media and forwards touch events.