PID that arrives with the data, either DATA0 or DATA1. When sending data, the host or
device sends alternating DATA0-DATA1 PIDs. By comparing the Data PID with the state
of the internal toggle bit, the host or device can detect a corrupted handshake packet.
SETUP tokens are unique to CONTROL transfers. They preface eight bytes of data from
which the peripheral decodes host Device Requests.
SOF tokens occur once per millisecond, denoting a USB frame.
There are three handshake PIDs: ACK, NAK, and STALL.
•
•
ACK means “success;” the data was received error-free.
NAK means “busy, try again.” It’s tempting to assume that NAK means “error,”
but it doesn’t. A USB device indicates an error by not responding.
•
STALL means that something unforeseen went wrong (probably as a result of mis-
communication or lack of cooperation between the software and firmware writers).
A device sends the STALL handshake to indicate that it doesn’t understand a
device request, that something went wrong on the peripheral end, or that the host
tried to access a resource that isn’t there. It’s like “halt,” but better, because USB
provides a way to recover from a stall.
A PRE (Preamble) PID precedes a low-speed (1.5 Mbps) USB transmission. The EZ-
USB family supports high-speed (12 Mbps) USB transfers only, so it ignores PRE packets
and the subsequent low-speed transfer.
1.5
Host is Master
This is a fundamental USB concept. There is exactly one master in a USB system: the
host computer. USB devices respond to host requests. USB devices cannot send informa-
tion between themselves, as they could if USB were a peer-to-peer topology.
Actually, 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. But that’s the only way to “yank the host’s chain.” Everything
else happens because 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
EZ-USB TRM v1.9
Chapter 1. Introducing EZ-USB
Page 1-5
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