PIC16C745/765
The Device descriptor provides general information
such as manufacturer, product number, serial number,
USB device class the product falls under, and the num-
ber of different configurations supported. There can
only be one Device descriptor for any given applica-
tion.
10.2
Application Isolation
Microchip provides a comprehensive support library of
standard chapter 9 USB commands. These libraries
provide a software layer to insulate the application
software from having to handle the complexities of the
USB protocol. A simple Put/Get interface is imple-
mented to allow most of the USB processing to take
place in the background within the USB interrupt ser-
vice routine. Applications are encouraged to use the
provided libraries during both enumeration and config-
ured operation.
The Configuration descriptor provides information on
the power requirements of the device and how many
different interfaces are supported when in this configu-
ration. There may be more than one configuration for
each device, (i.e., a high power device may also sup-
port a low power configuration).
10.3
Introduction
The Interface descriptor details the number of end-
points used in this interface, as well as the class driver
to use should the device support functions in more
than just one device class. There can only be one
Interface descriptor for each configuration.
The USB peripheral module supports Low Speed con-
trol and interrupt (IN and OUT) transfers. The imple-
mentation supports 3 endpoint numbers (0, 1, 2) for a
total of 6 endpoints.
The Endpoint descriptor details the actual registers for
a given function. Information is stored about the trans-
fer types supported, direction (In/Out), bandwidth
requirements and polling interval. There may be more
than one endpoint in a device, and endpoints may be
shared between different interfaces.
The following terms are used in the description of the
USB module:
• MCU - The core processor and corresponding
firmware
• SIE - Serial Interface Engine: That part of the
USB that performs functions such as CRC gener-
ation and clocking of the D+ and D- signals.
Many of the four descriptors listed above will reference
or index different String descriptors. String descriptors
are used to provide vendor specific or application spe-
cific information. They may be optional and are
encoded in “Unicode” format.
• USB - The USB module including SIE and regis-
ters
• Bit Stuffing - forces insertion of a transition on D+
and D- to maintain clock synchronization
10.1.7 DEVICE CLASSES/CLASS DRIVERS
• BD - Buffer Descriptor
Operating systems provide drivers which group func-
tions together by common device types called classes.
Examples of device classes include, but are not limited
to, storage, audio, communications and HID (Human
Interface). Class drivers for a given application are ref-
erenced in both the Device descriptor and Interface
descriptor. Most applications can find a Class Driver
which supports the majority of their function/command
needs. Vendors who have a requirement for specific
commands which are not supported by any of the
standard class drivers may provide a vendor specific
“.inf” file or driver for extra support.
• BDT - Buffer Descriptor Table
• EP - Endpoint (combination of endpoint number
and direction)
• IN - Packet transfer into the host
• OUT - Packet transfer out of the host
10.4
USB Transaction
When the USB transmits or receives data the SIE will
first check that the corresponding endpoint and direc-
tion Buffer Description UOWN bit equals 1. The USB
will move the data to or from the corresponding buffer.
When the TOKEN is complete, the USB will update the
BD status and change the UOWN bit to 0. The USTAT
register is updated and the TOK_DNE interrupt is set.
When the MCU processes the TOK_DNE interrupt it
reads the USTAT register, which gives the MCU the
information it needs to process the endpoint. At this
point the MCU will process the data and set the corre-
sponding UOWN bit. Figure 10-1 shows a time line of
how a typical USB token would be processed.
10.1.8 SUMMARY
While a complete USB overview is beyond the scope
of this document, a few key concepts must be noted.
Low speed communication is designed for devices,
which in the past, used an interrupt to communicate
with the host. In the USB scheme, devices do not
directly interrupt the processor when they have data.
Instead the host periodically polls each device to see if
they have any data. This polling rate is negotiated
between the device and host, giving the system a
guaranteed latency.
For more details on USB, see the USB V1.1 spec,
available from the USB website at www.usb.org.
DS41124A-page 58
Advanced Information
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
