Programmers Interface; Buffer Descriptor Table - NXP Semiconductors MKL27Z128VFM4 Reference Manual

Table of Contents

Programmers interface

Figure 33-6. Typical Device-only block diagram (bus-powered with external regulator)

33.4 Programmers interface

This section discusses the major components of the programming model for the USB

To efficiently manage USB endpoint communications USBFS implements a Buffer

Descriptor Table (BDT) in system memory. The BDT resides on a 512-byte boundary in

system memory and is pointed to by the BDT Page Registers. Every endpoint direction

requires two 8-byte Buffer Descriptor (BD) entries. Therefore, a system with 16 fully

bidirectional endpoints would require 512 bytes of system memory to implement the

BDT. The two BD entries allows for an EVEN BD and ODD BD entry for each endpoint

direction. This allows the microprocessor to process one BD while USBFS is processing

the other BD. Double buffering BDs in this way allows USBFS to transfer data easily at

the maximum throughput provided by USB.

Software should manage buffers for USBFS by updating the BDT when needed. This

allows USBFS to efficiently manage data transmission and reception, while the

microprocessor performs communication overhead processing and other function

dependent applications. Because the buffers are shared between the microprocessor and

USBFS, a simple semaphore mechanism is used to distinguish who is allowed to update

Place resistors close to the processor

KL27 Sub-Family Reference Manual , Rev. 5, 01/2016

External 5v to 3.3v regulator

USB connecttor

Freescale Semiconductor, Inc.