PIC16C745SLASHP datasheet(68/158 Pages) MICROCHIP

68 / 158 page

PIC16C745/765

DS41124A-page 68

Advanced Information

10.6

Buffer Descriptor Table (BDT)

To efficiently manage USB endpoint communications

the USB implements a Buffer Descriptor Table (BDT)

in register space. Every endpoint requires a 4 byte

Buffer Descriptor (BD) entry. Because the buffers are

shared between the MCU and the USB, a simple

semaphore mechanism is used to distinguish which is

allowed to update the BD and buffers in system mem-

ory. The UOWN bit is cleared when the BD entry is

“owned” by the MCU. When the UOWN bit is set to 1,

the BD entry and the buffer in system memory is

owned by the USB. The MCU should not modify the

BD or its corresponding data buffer.

The Buffer Descriptors provide endpoint buffer control

information for the USB and MCU. The Buffer Descrip-

tors have different meaning based on the value of the

UOWN bit.

The USB Controller uses the data stored in the BDs

when UOWN = 1 to determine:

•

Data0 or Data1 PID

•

Data toggle synchronization enable

•

Number of bytes to be transmitted or received

•

Starting location of the buffer

The MCU uses the data stored in the BDs when

UOWN = 0 to determine:

•

Data0 or Data1 PID

•

The received TOKEN PID

•

Number of bytes transmitted or received

Each endpoint has a 4 byte Buffer Descriptor and

points to a data buffer in the USB dual port register

space. Control of the BD and buffer would typically be

handled in the following fashion:

•

The MCU verifies UOWN = 0, sets the BDndAL to

point to the start of a buffer, if necessary fills the

buffer, then sets the BDndST byte to the desired

value with UOWN = 1.

•

When the host commands an in or out transac-

tion, the Serial Interface Engine (SIE) performs

the following:

- Get the buffer address

- Read or write the buffer

- Update the USTAT register

- Update the buffer descriptors with the packet

ID (PID) value

- Set the data 0/1 bit

- Update the byte count

- Clear the UOWN bit

•

The MCU is interrupted and reads the USTAT,

translates that value to a BD, where the UOWN,

PID, Data 0/1, and byte count values are checked.

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