Serial Port Complete - Latest Microcontroller projects

Chapter 14

data periodically. USB virtual COM-port devices use interrupt transfers to send

status information to the host. Isochronous transfers have guaranteed delivery

time but no error correcting. Real-time audio uses isochronous transfers. With

driver support, USB virtual COM-port devices can use isochronous transfers

for COM-port data in place of bulk transfers.

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Each USB transfer consists of one or more transactions (Table 14-2). Each

transaction contains a token packet, a data packet, and a handshake packet. (An

exception is isochronous transfers, which don’t have handshake packets.) Each

packet begins with a packet ID (PID). The function of the PID varies with the

packet type.

The token packet contains the device address and the endpoint number the

transaction is directed to. The token packet’s PID identifies the type of packet:

SETUP (the first packet in a control transfer), OUT (other host-to-device

packet), IN (device-to-host packet), or SOF (start-of-frame marker).

The data packet contains any data the host or device is sending in the transac-

tion. For control transfers, the transfer’s stage and the specific request determine

who sends the data. For other transfers, the endpoint’s direction determines

who sends the data. The PID contains the data-toggle value, explained below.

The receiver of the data packet (or the device if there is no data packet) sends

the handshake packet. The PID contains a code to indicate the status of the

transaction. ACK means success. NAK on an IN transaction means the device

has no data to send. NAK on an OUT transaction means the device was too

busy to accept the data sent. (The host can try again later.) STALL means the

device doesn’t support a received request in a control transfer or the endpoint’s

Table 14-1: Each of USB’s four transfer types suit different uses.

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Control Three stages (Setup, Data, Status). Enumeration. Set and get serial-port

parameters. Set serial-port control

signals.

Interrupt Guaranteed maximum latency. Status information.

Bulk Fastest on an otherwise idle bus. COM-port data.

Isochronous Guaranteed transfer rate but no error

detecting.

COM-port data (requires driver

support from vendor).