HCS08 datasheet(314/374 Pages) NXP

314 / 374 page

Universal Serial Bus Device Controller (S08USBV1)

MC9S08JM60 Series Data Sheet, Rev. 5

314

Freescale Semiconductor

17.4.2

Buffer Descriptor Table (BDT)

To efficiently manage USB endpoint communications, the USB module implements a buffer descriptor

table (BDT) comprised of buffer descriptors (BD) in the local USB RAM. The BD entries provide status

or control information for a corresponding endpoint. The BD entries also provide an address to the

endpoint’s buffer. A single BD for an endpoint direction requires 3-bytes. A detailed description of the

BDT format is provided in the next sections.

The software API intelligently manages buffers for the USB module by updating the BDT when needed.

This allows the USB module to efficiently handle data transmission and reception, while the

microcontroller performs communication overhead processing and other function dependent applications.

Because the buffers are shared between the microcontroller and the USB module, a simple semaphore

mechanism is used to distinguish who is allowed to update the BDT and buffers in buffer memory. A

semaphore bit, the OWN bit, is cleared to 0 when the BD entry is owned by the microcontroller. The

microcontroller is allowed read and write access to the BD entry and the data buffer when the OWN bit is

0. When the OWN bit is set to 1, the BD entry and the data buffer are owned by the USB module. The USB

module now has full read and write access and the microcontroller must not modify the BD or its

corresponding data buffer.

17.4.2.1

Multiple Buffer Descriptor Table Entries for a Single Endpoint

Every endpoint direction requires at least one three-byte Buffer Descriptor entry. Thus, endpoint 0, a

bidirectional control endpoint, requires one BDT entry for the IN direction, and one for the OUT direction.

Using two BD entries also allows for double-buffering. Double-buffering BDs allows the USB module to

easily transfer data at the maximum throughput provided by the USB module. Double buffering allows the

MCU to process one BD while the USB module is processing the other BD.

To facilitate double-buffering, two buffer descriptor (BD) entries are needed for each endpoint direction.

One BD entry is the EVEN BD and the other is the ODD BD.

17.4.2.2

Addressing Buffer Descriptor Table Entries

The BDT addressing is hardwired into the module. The BDT occupies the first portion of the USB RAM.

To access endpoint data via the USB or MCU, the addressing mechanism of the buffer descriptor table

must be understood.

All enabled IN and OUT endpoint BD entries are indexed into the BDT to allow easy access via the USB

module or the MCU. The figure below shows the USB RAM organization. The figure shows that the first

entries in the USB RAM are dedicated to storage of the BDT entries - i.e. the first 30 bytes of the USB

RAM (0x00 to 0x1D) are used to implement the BDT.

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