FT121Q-R datasheet(15/46 Pages) FTDI

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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

Buffer

5 (ISO)

1 (ISO)

Configuration 2

Configuration 0

Configuration 1

(ISO)

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