Motorola USB08 User Manual
Page 61
Universal Serial Bus (USB) Interface
Characteristics of the USB08 Reference Design
USB08 Evaluation Board
Designer Reference Manual
MOTOROLA
Universal Serial Bus (USB) Interface
61
The integrated USB module of the MC68HC908JB8 works at a data rate
of 1.5 Mbit/s, thus it is defined as a low-speed USB device. For
measuring and control applications, typically only small data rates are
needed, and this is already realized using low-speed USB devices. A
low-speed USB device ensures an information flow rate, which can be
compared roughly with an RS232 link with 9600 baud. At first glance,
that doesn’t seem to be much; however, the USB variant offers a set of
other advantages.
While RS232 always represents point-to-point connection, USB
supports a bus structure. The PC serves as bus master and several USB
devices can be attached. If the connections (ports) at the PC (host) are
not sufficient, USB hubs can be inserted. Hubs can be cascaded up to
five levels. Each USB device is addressed by the host via a unique
address. The address range supports up to 127 addresses. Thus, a
whole measurement and recording system can be arranged easily using
a dozen low-speed USB devices and two or three commercial hubs.
This reference design contains USB08 evaluation board firmware;
therefore, this Plug & Play demo application can be evaluated
immediately. Beyond that, all source code is provided in the form of C
modules for the M68HC08 Cosmic C compiler. The user can use these
sources as a starting point for their own USB development.
The most important modules for the USB implementation are the source
code modules
, and the header file
(see
Section 3. Software Module Descriptions
). The USB functional
description refers to these source code modules.
Administration of the USB device takes the largest portion of the
referenced implementation source code. This is done via so-called
standard device service requests. These transfers take place via the
control endpoint 0 and are multi-level, complex communication
procedures. Since the implementation of these complex functions
virtually can be transferred as a block from the reference design to any
other application, the practical work for the administration of a USB
device should not be overestimated. Simply, the actual data
communication functions (which take place using the interrupt endpoints
1 and 2) can be adapted to the concrete user application.