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FT121 Datasheet(PDF) 15 Page - Future Technology Devices International Ltd. |
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FT121 Datasheet(HTML) 15 Page - Future Technology Devices International Ltd. |
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15 / 46 page  Copyright © 2014 Future Technology Devices International Limited 15 Document No.: FT_000648 FT121 ENHANCED USB DEVICE CONTROLLER WITH SPI SLAVE IC Datasheet Version 1.2 Clearance No.: FTDI# 312 As the primary endpoint, EP2 is suitable for transmitting or receiving relatively large data. To improve the data throughput, EP2 is implemented with double buffering. This allows the concurrent operation between USB bus access and MCU local bus access. For example, for EP2 IN endpoint (EPI5), the USB host can read data from FT121 Buffer 0 while the local MCU is writing to Buffer 1 at the same time. The USB host can subsequently read from the FT121 Buffer 1 without waiting for it to be filled. Buffer switching is handled automatically by FT121. 5.2 Endpoint Buffer Management in Enhanced Mode In enhanced mode the FT121 supports a dedicated 1kB buffer for IN packets and a dedicated 1kB buffer for OUT packets. The OUT/IN buffer can be allocated to any endpoint with the same direction, up to a maximum of 504 bytes double buffered (1008 bytes in total) to one endpoint. 504 is the maximum byte count as there are 1024 bytes in total per OUT/IN Buffer and 8 bytes for IN and OUT packets on control endpoint 0 must always be reserved. Control, interrupt and bulk endpoints can have a maximum packet size of 64 bytes and only isochronous endpoints can be allocated more than 64 bytes. Isochronous modes can have larger buffer sizes as USB packets can be larger than 64 bytes for isochronous transfer. Isochronous buffers are managed in the same way as bulk, interrupt and control buffers – i.e. a buffer is for one USB packet only and will not span more than one USB packet. An example of buffer configurations follows, where Configuration 1 and 2 have larger isochronous buffers. Table 5.3 Example buffer configuration EP Buffer EP Buffer EP Buffer 7 1 7 0 6 1 6 0 5 (ISO) 1 5 1 6 1 5 0 6 0 4 1 2 1 4 0 2 0 3 1 3 0 2 1 5 (ISO) 0 2 0 1 1 1 0 0 1 0 1 0 1 0 0 0 0 0 0 0 1 1 (ISO) 0 1 1 (ISO) Configuration 2 Configuration 0 Configuration 1 7 (ISO) 7 (ISO) |
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