Copyright © 2013 Future Technology Devices International Limited

8

DS_FT312D USB ANDROID HOST IC Datasheet

Version 1.1

Document No.: FT_000816 Clearance No.: FTDI# 331

4 Function Description

The FT312D is FTDI’s second dedicated Android Open Accessory Mode integrated circuit device or Android

Host. The FT312D USB host port is dedicated to support of the Android Open Accessory class and will

bridge data between this port and the UART interface.

4.1 Key Features

Easy to use Android Open Accessory IC translating the Device port of the Android device into UART

capabilities

4.2 Functional Block Descriptions

The following paragraphs describe each function within FT312D. Please refer to the block diagram shown

inFigure 2.1.

4.2.1 UART Interface Module

The FT312DUART module controls the UART interface providing basic RXD, TXD signalling with

RTS#/CTS# hardware flow control. An additional TX_Active signal is supplied to control external RS485

transceivers for users wishing to create a USB to RS485 bridge. The UART supports baud rates from 300

baud to 921600 baud. A full description of the UART module is provided in Section 5.

4.2.2 Buffers

The FT312D provides internal buffering between the USB port and the UART port of the IC for smooth

data streaming.

The Android device can send NAK’s to the USB OUT token sent from the FT312D. This can happen when

the UART application on the Android is running in the background or multiple applications are launched in

the Android device. When the UART application on the Android device is not accepting data, the data will

be buffered in the UART RX buffer in FT312D. The UART RX buffer size is 5512 bytes.

The UART_TX buffer which stores data from the USB port, heading for the UART is 256 bytes.

4.2.3 USB Host

The USB Host block handles the parallel-to-serial and serial-to-parallel conversion of the USB physical

layer. This includes bit stuffing, CRC generation.

4.2.4 USB Transceivers

USB transceiver cells provide the physical USB device interface supporting USB 1.1 and USB 2.0

standards. Low-speed and full-speed USB data rates are supported. The output driver provides 3V3 level

slew rate control signalling, whilst a differential receiver and two single ended receivers provide USB

DATA IN, SE0 and USB Reset condition detection. These cells also include integrated internal pull-down

resistors as required for host mode.

4.3 Default Descriptor Strings

When the USB port is connected to the Android USB port, the Android platform will determine which

application to load based on the strings read from the FT312D. These strings are configurable with a

Windows utility: FT312D_Cofiguration available for download from the FTDI website.

http://www.ftdichip.com

Please refer to the application note AN_236 User Guide for FT312D Configuration to change the default

string values.