Overview

UM0463

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There are two interrupt sources that inform the software about the USB status, generic USB

interrupt and the End of Suspend interrupt. The generic USB interrupt integrates six USB

events. They are defined in Table 3.

The End Of Suspend (ESUSP) interrupt is asserted to wake up the device when bus activity

is detected in suspend mode.

In summary, to send data from the device to the host, one has to copy the data to one of the

IN endpoint buffers, then set the number of bytes to be sent in the corresponding endpoint

counter register, and set the endpoint status register to ACK state. Afterward, the device

waits for the USB host to send an IN token to that endpoint and the device sends the data

bytes after the IN token. After the host receives the data packet, it sends an ACK token to

acknowledge reception. The device sees the ACK token from the host and generates a CTR

interrupt to inform the software that the data transmission is successful.

Contrary to receiving data from the host, the corresponding endpoint counter register has to

be set as the number of bytes expected and the endpoint status register is set to ACK state.

When the host sends an OUT token to that endpoint and followed by the data packet, the

device copies the data packet to the endpoint DMA buffer. After the device receives the data

packet correctly, it sends an ACK token to acknowledge reception and then generates a

CTR interrupt to inform the software that the data has been successfully received. At this

time, the counter register contains the difference of expected bytes and received bytes.

If the device wants to send or receive data bytes containing more than the maximum packet

size of an endpoint, several sending or receiving procedures must be performed.

The endpoint responds with NAK to any IN token for IN direction or NAK to any OUT

token for OUT direction, and without data transfer.

ACK

– IN direction: The endpoint can send data to the host and the counter register gives

the number of bytes to be sent.

– OUT direction: The endpoint can receive data and responses with ACK after the

data packet and the counter register indicates the number of bytes received after the

device ACKs the data packet.

At the end of the data transmission, the hardware machine switches to NAK.

1.

In case of error on data toggling bit, the hardware machine does the data retransmission

Table 2.

Endpoint status (continued)

Status

Meaning

Table 3.

USB events

Events

Meaning

Correct Transfer (CTR)

A correct USB transfer is complete on either endpoint. The software is responsible for

checking what the endpoint is.

Setup Overrun (SOVR)

A SETUP transfer is complete when a previous CTR is pending. It happens when the

software does not have enough time to process a CTR event.

Error (ERR)

The USB cell detects a bus error. Register ERRSR gives the type of error.

Suspend request (SUSP)

The USB cell detects that the bus is idle for more than 3ms, requiring the device to enter

suspend mode.

Reset (RESET)

The USB cell detects a USB reset sequence on the bus.

Start of frame (SOF)

A SOF token is received.