PIC16C765SLASHP datasheet(57/158 Pages) MICROCHIP

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Advanced Information

DS41124A-page 57

PIC16C745/765

10.0

UNIVERSAL SERIAL BUS

10.1

Overview

This section introduces a minimum amount of informa-

tion on USB. If you already have basic knowledge of

USB, you can safely skip this section. If terms like

Enumeration, Endpoint, IN/OUT Transactions, Trans-

fers and Low Speed/Full Speed are foreign to you,

read on.

USB was developed to address the increased connec-

tivity needs of PC’s in the PC 2000 specification.

There was a base requirement to increase the band-

width and number of devices, which could be attached.

Also desired were the ability for hot swapping, user

friendly operation, robust communications and low

cost. The primary promoters of USB are Intel, Com-

paq, Microsoft and NEC.

USB is implemented as a Tiered Star topology, with

the host at the top, hubs in the middle, spreading out

to the individual devices at the end. USB is limited to

127 devices on the bus, and the tree cannot be more

than 6 levels deep.

USB is a host centric architecture. The host is always

the master. Devices are not allowed to “speak” unless

“spoken to” by the host.

Transfers take place at one of two speeds. Full Speed

is 12 Mb/s and Low Speed is 1.5 Mb/s. Full Speed

covers the middle ground of data intensive audio and

compressed video applications, while low speed sup-

ports less data intensive applications.

10.1.1

TRANSFER PROTOCOLS

Four

transfer

protocols

are

defined,

each

with

attributes:

- Isochronous Transfers, meaning equal time,

guarantee a fixed amount of data at a fixed

rate. This mode trades off guaranteed data

accuracy for guaranteed timeliness. Data

validity is not checked because there isn’t

time to re-send bad packets anyway and the

consequences of bad data are not cata-

strophic.

- Bulk Transfers are the converse of Isocho-

nous. Data accuracy is guaranteed, but time-

liness is not.

- Interrupt Transfers are designed to communi-

cate with devices which have a moderate

data rate requirement. Human Interface

Devices like keyboards are but one example.

For Interrupt Transfers, the key is the desire

to transfer data at regular intervals. USB peri-

odically polls these devices at a fixed rate to

see if there is data to transfer.

- Control Transfers are used for configuration

purposes.

10.1.2

FRAMES

Information communicated on the bus is grouped in a

format called Frames. Each Frame is 1 ms in duration

and is composed of multiple transfers. Each transfer

type can be repeated more than once within a frame.

10.1.3

POWER

Power has always been a concern with any device.

With USB, 5 volt power is now available directly from

the bus. Devices may be self-powered or bus-pow-

ered. Self-powered devices will draw power from a

wall adapter or power brick. On the other hand, bus-

powered devices will draw power directly from the

USB bus itself. There are limits to how much power

can be drawn from the USB bus. Power is expressed

in terms of “unit loads” (

≤100 mA). All devices, includ-

ing Hubs, are guaranteed at least 1 unit load (low

power), but must negotiate with the host for up to 5

unit loads (high power). If the host determines that the

bus as currently configured cannot support a device’s

request for more unit loads, the device will be denied

the extra unit loads and must remain in a low power

configuration.

10.1.4

END POINTS

At the lowest level, each device controls one or more

endpoints. An endpoint can be thought of as a virtual

port. Endpoints are used to communicate with a

device’s functions. Each endpoint is a source or sink of

data. Endpoints have both an In and Out direction

associated with it. Each device must implement end-

point 0 to support Control Transfers for configuration.

There are a maximum of 15 endpoints available for

use by each full speed device and 6 endpoints for

each slow speed device. Remember that the bus is

host centric, so In/Out is with respect to the host and

not the device.

10.1.5

ENUMERATION

Prior to communicating on the bus, the host must see

that a new device has been connected and then go

through an “enumeration process”. This process

allows the host to ask the device to introduce itself,

and negotiate performance parameters, such as

power consumption, transfer protocol and polling rate.

The enumeration process is initiated by the host when

it detects that a new device has attached itself to the

bus. This takes place completely in the background

from the application process.

10.1.6

DESCRIPTORS

The USB specification requires a number of different

descriptors to provide information necessary to iden-

tify a device, specify its endpoints, and each endpoint’s

function. The five general categories of descriptors are

Device, Configuration, Interface, End Point and String.

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