PIC16C765SLASHSP datasheet(58/158 Pages) MICROCHIP

58 / 158 page

PIC16C745/765

DS41124A-page 58

Advanced Information

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.

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).

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 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.

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.

10.1.7

DEVICE CLASSES/CLASS DRIVERS

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.

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.

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.

10.3

Introduction

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 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.

• USB - The USB module including SIE and regis-

ters

• Bit Stuffing - forces insertion of a transition on D+

and D- to maintain clock synchronization

• BD - Buffer Descriptor

• 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

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.

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