PIC16C765SLASHP datasheet(72/158 Pages) MICROCHIP

72 / 158 page

PIC16C745/765

DS41124A-page 72

Advanced Information

ter must be provided for in each bank. The 16C745/

765 can save the appropriate registers in Common

RAM and not have to waste a byte in each bank for W

10.9.3.6

Buffer allocation

The PIC16C745/765 has 64 bytes of Dual Port RAM.

24 are used for the Buffer Descriptor Table (BDT) leav-

ing 40 bytes for buffers.

Endpoint 0 IN and OUT need dedicated buffers since a

setup transaction can never be NAKed. That leaves

three buffers for four possible Endpoints. But the USB

spec requires that low speed devices are only allowed

2 endpoints (USB 1.1 paragraph 5.3.1.2), where an

endpoint is a simplex connection that defined by the

combination of Endpoint number and direction.

The default configuration allocates individual buffers to

EP0 OUT, EP0 In, EP1 Out, and EP1 In.

The last

buffer is shared between EP2 In and EP2 Out. Again,

the spec says low speed devices can only use 2 end-

points beyond EP0. This configuration supports most

of the possible combinations of endpoints (EP1 OUT

and EP1 IN, EP1OUT and EP2IN, EP1 OUT and EP2

OUT, EP1 IN and EP2 OUT, EP1 IN and EP2 IN). The

only combination that is not supported by this configu-

ration is Endpoint 2 IN and Endpoint 2 OUT. If your

application needs both EP2 IN and EP2 OUT, the func-

tion USBReset will need to be edited to give each of

these dedicated buffers at the expense of EP1.

10.9.4

FUNCTION CALL REFERENCE

Interface between the Application and Protocol layer

takes place in three main functions: InitUSB, PutUSB

and GetUSB.

InitUSB should be called by the main program imme-

diately upon power-up. It sets up the Buffer Descriptor

Table, transitions the part to the Powered state, and

prepares the device for enumeration. At this point the

USB Reset is the only USB interrupt allowed, prevent-

ing the part from responding to anything on the bus

until it’s been reset. The USB Reset interrupt transi-

tions the part to the default state where it responds to

commands on address zero. When it receives a SET

ADDRESS command, the device transitions to the

addressed state and now responds to commands on

the new address.

PutUSB (Buffer pointer, Buffer size, Endpoint) sends

data up to the host. The pointer to the block of data to

transmit, is in the FSR/IRP, and the block size and

endpoint is passed in W register. If the IN buffer is

available for that endpoint, the block of data is copied

to the buffer, then the Data 0/1 bit is flipped and the

owns bit is set.

A buffer not available would occur

when it has been previously loaded and the host has

not requested that the USB peripheral transmit it. In

this case, a failure code would be returned so the

application can try again later.

GetUSB (Buffer Pointer, Endpoint) returns data sent

from the host. If there is a buffer ready (i.e., data has

been received from the host) it is copied to the desti-

nation pointed to by FSR/IRP (A buffer pointer in FSF/

IRP and the endpoint number in W must be provided.).

If no data is available, it returns a failure code. Thus,

the functions of polling for buffer ready and copying the

data are combined into the one function.

ServiceUSBInt handles all interrupts generated by the

USB peripheral.

First it copies the active buffer to

common RAM which provides a quick turn around on

the buffer in dual port RAM and also to avoids having

to switch banks during processing of the buffer.

StallUSBEP/UnstallUSBEP sets or clears the stall bit

in the endpoint control register. The stall bit indicates

to the host that user intervention is required and until

such intervention is made, further attempts to commu-

nicate with the endpoint will not be successful. Once

the user intervention has been made, UnstallUSBEP

will clear the bit allowing communications to take

place. These calls are useful to signal to the host that

user intervention is required. An example of this might

be a printer out of paper.

CheckSleep Tests the UCTRL.UIDLE bit if set, indi-

cating that there has been no activity on the bus for 3

mS, puts the device to sleep. This puts the part into a

low power standby mode until awakened by bus activ-

ity. This has to be handled outside the ISR because we

need the interrupt to wake us from sleep, and also

because the application may not be ready to sleep

when the interrupt occurs.

Instead, the application

should periodically call this function to poll the bit when

the device is in a good place to sleep.

Prior to putting the device to sleep, it enables the activ-

ity interrupt so the device will be awakened by the first

transition on the bus.

The device will immediately

jump to the ISR, recognizing the activity interrupt,

which then disables the interrupt and resumes pro-

cessing with the instruction following the CheckSleep

call.

ConfiguredUSB (Macro) Continuously polls the enu-

meration status bits and waits until the device has

been configured by the host.

10.9.5

BEHIND THE SCENES

The ISR calls ServiceUSBInt, which then further has

to mask the USB Interrupt register with the USB Inter-

rupt Enable bits, then see what caused the interrupt.

InitUSB only enables the Reset interrupt (USB_RST).

This prevents the device from responding to anything

on the bus until it’s been reset by the host. When the

reset is received, the Buffer Descriptors are initialized,

most of the rest of the interrupts are unmasked and

the

device

transitions

from

the

POWERED

DEFAULT state. Now it can respond to commands on

address zero. From there the rest of the enumeration

process takes place, including assigning an address to

the device through the SET_ADDRESS command and

selecting

configuration

through

the

PIC16C765/P Datasheet(PDF) 72 Page - Microchip Technology

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