Nxp Semiconductors UM10237 用户手册

UM10237_2

© NXP B.V. 2008. All rights reserved.

endpoints, the next packet will be received irrespective of whether the buffer has been 
cleared.  Any data not read from the buffer before the end of the frame is lost.  See 

 for more details.

If the software clears RD_EN before the entire packet is read, reading is terminated, and 
the data remains in the endpoint’s buffer. When RD_EN is set again for this endpoint, the 
data will be read from the beginning.

When writing data to an endpoint buffer, WR_EN (

) is set and software writes to the number of bytes it is going to 

send in the packet to the USBTxPLen register (

). It can then write data 

continuously in the USBTxData register.

When the the number of bytes programmed in USBTxPLen have been written to 
USBTxData, the WR_EN bit is cleared, and the TxENDPKT bit is set in the USBDevIntSt 
register. Software issues a Validate Buffer (

) command. The endpoint is now ready to send the packet. For IN 

isochronous endpoints, the data in the buffer will be sent only if the buffer is validated 
before the next FRAME interrupt occurs; otherwise, an empty packet will be sent in the 
next frame. If the software clears WR_EN before the entire packet is written, writing will 
start again from the beginning the next time WR_EN is set for this endpoint.

Both RD_EN and WR_EN can be high at the same time for the same logical endpoint. 
Interleaved read and write operation is possible.

In DMA mode, the DMA transfers data between RAM and the endpoint buffer.

The following sections discuss DMA mode operation. Background information is given in 
sections 

 and 

. The fields of the DMA Descriptor are described in section 

. The last three sections describe DMA operation: 

, and 

Within this section three types of transfers are mentioned:

1. USB transfers – transfer of data over the USB bus. The USB 2.0 specification refers 

to these simply as transfers. Within this section they are referred to as USB transfers 
to distinguish them from DMA transfers. A USB transfer is composed of transactions. 
Each transaction is composed of packets.

2. DMA transfers – the transfer of data between an endpoint buffer and system memory 

(RAM).

3. Packet transfers – in this section, a packet transfer refers to the transfer of a packet of 

data between an endpoint buffer and system memory (RAM). A DMA transfer is 
composed of one or more packet transfers.