Thursday, August 19, 2004
Interrupt Transfers
Any one who has had experience of interrupt requests on microcontrollers will know that interrupts are device generated. However under USB if a device requires the attention of the host, it must wait until the host polls it before it can report that it needs urgent attention!
Interrupt Transfers
->Guaranteed Latency
->Stream Pipe - Unidirectional
->Error detection and next period retry.
Interrupt transfers are typically non-periodic, small device "initiated" communication requiring bounded latency. An Interrupt request is queued by the device until the host polls the USB device asking for data.
->The maximum data payload size for low-speed devices is 8 bytes.
->Maximum data payload size for full-speed devices is 64 bytes.
->Maximum data payload size for high-speed devices is 1024 bytes
The above diagram shows the format of an Interrupt IN and Interrupt OUT transaction.
->IN: The host will periodically poll the interrupt endpoint. This rate of polling is specified in the endpoint descriptor which is covered later. Each poll will involve the host sending an IN Token. If the IN token is corrupt, the function ignores the packet and continues monitoring the bus for new tokens.
If an interrupt has been queued by the device, the function will send a data packet containing data relevant to the interrupt when it receives the IN Token. Upon successful recept at the host, the host will return an ACK. However if the data is corrupted, the host will return no status. If on the other hand a interrupt condition was not present when the host polled the interrupt endpoint with an IN token, then the function signals this state by sending a NAK. If an error has occurred on this endpoint, a STALL is sent in reply to the IN token instead.
->OUT: When the host wants to send the device interrupt data, it issues an OUT token followed by a data packet containing the interrupt data. If any part of the OUT token or data packet is corrupt then the function ignores the packet. If the function's endpoint buffer was empty and it has clocked the data into the endpoint buffer it issues an ACK informing the host it has successfully received the data. If the endpoint buffer is not empty due to processing of a previous packet, then the function returns an NAK. However if an error occurred with the endpoint consequently and its halt bit has been set, it returns a STALL.
Interrupt Transfers
->Guaranteed Latency
->Stream Pipe - Unidirectional
->Error detection and next period retry.
Interrupt transfers are typically non-periodic, small device "initiated" communication requiring bounded latency. An Interrupt request is queued by the device until the host polls the USB device asking for data.
->The maximum data payload size for low-speed devices is 8 bytes.
->Maximum data payload size for full-speed devices is 64 bytes.
->Maximum data payload size for high-speed devices is 1024 bytes
The above diagram shows the format of an Interrupt IN and Interrupt OUT transaction.
->IN: The host will periodically poll the interrupt endpoint. This rate of polling is specified in the endpoint descriptor which is covered later. Each poll will involve the host sending an IN Token. If the IN token is corrupt, the function ignores the packet and continues monitoring the bus for new tokens.
If an interrupt has been queued by the device, the function will send a data packet containing data relevant to the interrupt when it receives the IN Token. Upon successful recept at the host, the host will return an ACK. However if the data is corrupted, the host will return no status. If on the other hand a interrupt condition was not present when the host polled the interrupt endpoint with an IN token, then the function signals this state by sending a NAK. If an error has occurred on this endpoint, a STALL is sent in reply to the IN token instead.
->OUT: When the host wants to send the device interrupt data, it issues an OUT token followed by a data packet containing the interrupt data. If any part of the OUT token or data packet is corrupt then the function ignores the packet. If the function's endpoint buffer was empty and it has clocked the data into the endpoint buffer it issues an ACK informing the host it has successfully received the data. If the endpoint buffer is not empty due to processing of a previous packet, then the function returns an NAK. However if an error occurred with the endpoint consequently and its halt bit has been set, it returns a STALL.
