PIC16C765SLASHP datasheet(71/158 Pages) MICROCHIP

71 / 158 page

Advanced Information

DS41124A-page 71

PIC16C745/765

10.9

USB Firmware Users Guide

10.9.1

INTRODUCING THE USB SOFTWARE

INTERFACE

Microchip provides a layer of software that handles the

lowest level interface so your application won’t have to.

This provides a simple Put/Get interface for communi-

cation. Most of the USB processing takes place in the

background through the Interrupt Service Routine.

From the application viewpoint, the enumeration pro-

cess and data communication takes place without fur-

ther interaction.

FIGURE 10-2: USB SOFTWARE INTERFACE

10.9.2

INTEGRATING USB INTO YOUR

APPLICATION

The latest version of the USB interface software is

available on Microchip Technology’s website. See

http://www.microchip.com/

Communicating on USB is similar to communicating

via a hardware USART. The main difference is that a

USART typically works on a single byte at a time,

where USB operates on a buffer of up to 8 bytes at a

time.

There is one function defined to start the enumeration

process and two additional functions are defined for

moving buffers between the main application and the

USB peripheral. InitUSB initializes the USB peripheral

allowing the host to enumerate the device. Then for

normal data communications, function PutUSB sends

data to the host and function GetUSB receives data

from the host.

There's a lot that happens behind the scenes to make

the communication work, but these calls are all an

application needs to communicate on the bus. The

rest is handled on an interrupt basis.

InitUSB initializes the Buffer Descriptor table, and

enables the USB interrupt so enumeration can begin.

The actual enumeration process occurs automatically,

driven by the host and interrupt service routine. The

macro ConfiguredUSB waits until the device is in the

CONFIGURED mode and ready to go.

The time

required to enumerate is completely dependent on the

host and bus loading.

10.9.3

INTERRUPT STRUCTURE CONCERNS

10.9.3.1

Processor Resources

Most of the USB processing occurs via the interrupt

and thus is invisible to application. However it still con-

sumes processor resources.

These include ROM,

RAM, Common RAM, Stack Levels and processor

cycles. This section attempts to quantify the impact on

each of these resources, and shows ways to avoid

conflicts.

These are the considerations you'll need to take into

account if you write your own Interrupt Service Rou-

tine: Save W, Status, FSR and PCLATH which are the

file registers that may be corrupted by servicing the

USB interrupt.

We provide a skeleton ISR which will do this for you,

and includes tests for each of the possible ISR bits.

This provides a good place to start from if you haven't

already written your own. See file USB_INT.ASM.

10.9.3.2

Stack Levels

The hardware stack on the device is only 8 levels

deep. So the worst case call between the application

and ISR can only be 8 levels. The enumeration pro-

cess requires 6 levels, so it's best if the main applica-

tion holds off on any processing until enumeration is

complete. ConfiguredUSB is a macro that waits until

the enumeration process is complete for exactly this

purpose.

10.9.3.3

ROM

The code required to support the USB interrupt,

including the chapter 9 interface calls, but not including

the descriptor tables is about 1kW. The descriptor and

string descriptor tables can each take up to an addi-

tional 256W.

The location of these parts is not

restricted, and the linker script may be edited to control

the placement of each part.

See the Strings and

Descriptors sections in the linker script

10.9.3.4

RAM

With the exception of Common RAM discussed below,

servicing the USB interrupt costs ~40 bytes of RAM in

Bank 2. That leaves all the General Purpose RAM in

banks zero and one, plus half of bank two available for

your application to use.

10.9.3.5

Common RAM usage

The PIC16C745/765 has 16 bytes of common RAM.

These are the last 16 addresses in each bank and all

refer to the same 16 bytes of memory without regard

to which register bank is currently addressed by the

RP0 and RP1 bits.

These are particularly useful when responding to inter-

rupts.

When an interrupt occurs, the ISR doesn't

immediately know which bank is addressed.

With

devices that don't support common RAM, the W regis-

Main Application

Put

Get

Init

USB Peripheral

USB

PIC16C765/P Datasheet(PDF) 71 Page - Microchip Technology

PIC16C765/P Datasheet(HTML) 71 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL