TX4939XBG-400 Toshiba, TX4939XBG-400 Datasheet - Page 510

• Manufacturer Part Number: TX4939XBG-400
• Manufacturer: Toshiba

Specifications of TX4939XBG-400

• Cpu Core: TX49/H4 90nm
• Clock Mhz/max Mips: 400/520
• Inst./data Cache: 32KB (4 Way)/32KB (4 Way)
• Tlb: x
• 1cycle Mac: x
• Volts (v): 1.25/2.5/3.3
• Peripherals: DDR, NAND, ATA, ETHERNET, SECURITY, FPU, MMU, SPI, I2S, I2C, PCI, VIDEO, UART, TIMER, RTC
• Companion Chip: TC86C001FG
• Package: PGBA456

EMAC
18.3.7.2. Buffer Descriptor
Each buffer descriptor consists of a pointer to the data buffer, a control byte, a status byte, and a 2-byte Buffer Length
field. Table 18-4 indicates the buffer descriptor format.
When a buffer is listed in the buffer list, the BuffData field indicates the buffer head and the BuffLength field indicates the
size of buffers allocated as unused buffers. When the buffer is used in the Transmission queue or Reception queue, the
BuffData field indicates the descriptor head and the BuffLength field indicates the data length.
When a buffer is placed in the buffer list, the system software is responsible for allocating the Buffer Length field and
setting the size.
Similar to the frame descriptor, usage of the BDCtl field or BDStat field varies depending on the queue type.
18.3.7.2.1. BDCtl field (buffer descriptor control)
The transmission queue does not use the BDCtl field. In the case of the reception queue, the number of buffer
descriptors per buffer area is set in the BDCtl field. The first buffer descriptor in a frame is number 0, and the subsequent
buffer descriptors are number 1 and number 2.
In an empty buffer list, the BDCtl field is used to record buffer ownership. This makes it possible to allocate the buffer
descriptor and synchronize release of the buffer. This also prevents the Ethernet Controller from using the buffer in a loop
before the system empties the buffer.
Rev. 3.1 November 1, 2005
Bit(s)
7
6 : 0
Note:
Mnemonic
COwnsBD
RxBDSeqN
As shown in step 4 of 18.3.8.4 Frame transmission procedure, more than 8 bytes of data is required as a
Transmission packet data. One Transmission packet cannot consist of multiple data buffers, so you have
to implement your design so the BuffLength (buffer length) total is 8 bytes or more.
BuffData
BDCtl
BuffData
BDCtl
BDStat
Buff Length
Byte 3
Field Name
Buffer Descriptor
Owner
Reception Buffer
Descriptor
Number
Buffer Data Pointer
Buffer Descriptor Control
Buffer Descriptor Status
Buffer Length
BDStat
Byte 2
Table 18-4 Buffer Descriptor Format
Description
COwnsBD
1: The Ethernet Controller owns the buffer descriptor. When the system sets
0: The system owns the buffer descriptor. When the Ethernet Controller
RxBDSeqN
After receiving data, this field indicates the running number of this buffer in the
current buffer area.
the COwnsBD bit, the buffer can freely receive data.
clears the COwnsBD bit, it indicates that the buffer is full.
Figure 18-8 BDCtl Field
BuffLength
18-18
Byte 1
COwnsBD
Address of the buffer that stores data (32 bits)
Control of this buffer descriptor
Status of this buffer descriptor
Field that expresses the length of this buffer
7
6
Byte 0
RxBDSeqN
00h
04h
Toshiba RISC Processor
Offset
TX4939
0
18
18