Several application notes for FTDI devices can be found in this section of the Knowledgebase. All of those available here and some additional ones are available on the Application Notes page in the Documents section of the FTDI website. This page contains the complete current application note library for download as PDF files.
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The application notes available here are:
AN232R-01 Bit Bang Modes for the FT232R and FT245R
AN232R-02 FTDIChip-ID for the FT232R and FT245R
AN2232C-01 Command Processor for MPSSE and MCU Host Bus Emulation Modes
AN2232C-02 Bit Mode Functions for the FT2232C