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Chapter 1. Introducing EZ-USB FX2 Page 1-3

1.4 Host Is Master

This is a fundamental USB concept. There is exactly one master in a USB system: the host com-

puter. USB devices respond to host requests.

USB devices cannot send information among

themselves, as they could if USB were a peer-to-peer topology.

However, there is one case where a USB device can initiate signaling without prompting from the

host. After being put into a low-power “suspend” mode by the host, a device can signal a “remote

wakeup”. This is the only case in which the USB device is the initiator; in all other cases, the host

makes device requests and the device responds to them.

There’s an excellent reason for this host-centric model. The USB architects were keenly mindful of

cost, and the best way to make low-cost peripherals is to put most of the “smarts” into the host

side, the PC. If USB had been defined as peer-to-peer, every USB device would have required

more intelligence, raising cost.

1.5 USB Direction

Because the host is always the bus master, it’s easy to remember USB direction: OUT means from

the host to the device, and IN means from the device to the host. FX2 nomenclature uses this

naming convention. For example, an endpoint that sends data to the host is an IN endpoint. This

can be confusing at first, because the FX2

sends

data to the host by loading an IN endpoint buffer.

Likewise, the FX2

receives

host data from an OUT endpoint buffer.

1.6 Tokens and PIDs

In this manual, you’ll read statements such as: “When the host sends an IN token…,” or “The

device responds with an ACK”. What do these terms mean?

A USB transaction consists of data packets identified by special codes called Packet IDs or PIDs.

A PID signifies what kind of packet is being transmitted. There are four PID types, shown in

Table 1-1.

Table 1-1. USB PIDS

PID Type PID Name

Token IN, OUT, SOF, SETUP

Data DATA0, DATA1, DATA2, MDATA

Handshake ACK, NAK, STALL, NYET

Special PRE, ERR, SPLIT, PING

Bold type indicates PIDs introduced with USB 2.0

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Cypress Semiconductor FX2LP Technical Information Seite 29