mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 952
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Mcf51jf128 Reference Manual
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Freescale Semiconductor, Inc
Datasheet
1.MCF51JF128.pdf
(1339 pages)
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Programmers Interface
39.3.3 Addressing Buffer Descriptor Table Entries
An understanding of the addressing mechanism of the Buffer Descriptor Table is useful
when accessing endpoint data via the USB-FS or microprocessor. Some points of interest
are:
When a USB token on an enabled endpoint is received, the USB-FS uses its integrated
DMA controller to interrogate the BDT. The USB-FS reads the corresponding endpoint
BD entry to determine if it owns the BD and corresponding buffer in system memory.
To compute the entry point in to the BDT, the BDT_PAGE registers is concatenated with
the current endpoint and the TX and ODD fields to form a 32-bit address. This address
mechanism is shown in the following diagrams:
39.3.4 Buffer Descriptor Formats
The Buffer Descriptors (BD) provide endpoint buffer control information for the USB-FS
and microprocessor. The Buffer Descriptors have different meaning based on whether it
is the USB-FS or microprocessor reading the BD in memory.
952
BDT_PAGE
END_POINT
TX
ODD
Field
• The Buffer Descriptor Table occupies up to 512 bytes of system memory.
• 16 bidirectional endpoints can be supported with a full BDT of 512 bytes.
• 16 bytes are needed for each USB endpoint direction.
• Applications with less than 16 endpoints require less RAM to implement the BDT.
• The BDT Page Registers point to the starting location of the BDT.
• The BDT must be located on a 512-byte boundary in system memory.
• All enabled TX and RX endpoint BD entries are indexed into the BDT to allow easy
access via the USB-FS or MCU core.
Table 39-1. Data Direction for USB Host or USB Target (continued)
Host
BDT_PAGE registers in the Control Register Block
END POINT field from the USB TOKEN
1 for an TX transmit transfers and 0 for an RX receive transfers
This bit is maintained within the USB-FS SIE. It corresponds to the buffer currently in use. The
buffers are used in a ping-pong fashion.
Description
Table 39-2. BDT Address Calculation Fields
MCF51JF128 Reference Manual, Rev. 2, 03/2011
Rx
IN
Preliminary
Out or Setup
Freescale Semiconductor, Inc.
Tx
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