DS31256 datasheet(82/183 Pages) MAXIM

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DS31256 256-Channel, High-Throughput HDLC Controller

82 of 183

Bit 14/Indirect Access Read/Write (IARW). When the host wishes to read data from the internal transmit low-

watermark RAM, this bit should be written to 1 by the host. This causes the device to begin obtaining the data

from the channel location indicated by the HCID bits. During the read access, the IAB bit is set to 1. Once the data

is ready to be read from the TFLWM register, the IAB bit is set to 0. When the host wishes to write data to the

internal transmit low-watermark RAM, this bit should be written to 0 by the host. This causes the device to take

the data that is currently present in the TFLWM register and write it to the channel location indicated by the HCID

bits. When the device has completed the write, the IAB is set to 0.

Bit 15/Indirect Access Busy (IAB). When an indirect read or write access is in progress, this read-only bit is set

to 1. During a read operation, this bit is set to 1 until the data is ready to be read. It is set to 0 when the data is

ready to be read. During a write operation, this bit is set to 1 while the write is taking place. It is set to 0 once the

write operation has completed.

TFLWM

Transmit FIFO Low Watermark

09A4h

Bit #

Name

TLWM7

TLWM6

TLWM5

TLWM4

TLWM3

TLWM2

TLWM1

TLWM0

Default

Bit #

Name

n/a

TLWM9

TLWM8

Default

Note: Bits that are underlined are read-only; all other bits are read-write.

Bits 0 to 9/Low Watermark (TLWM0 to TLWM9). These bits indicate the setting of the transmit low

watermark. The low watermark setting is the number of blocks left in the transmit FIFO before the DMA gets

more data from the PCI bus. The low-watermark setting must be between (inclusive) one block and one less than

the number of blocks in the link-list chain for the particular channel involved. For example, if five blocks are

linked together, the low watermark can be set to 1, 2, or 3. In other words, the low watermark can be set at a value

of 1 to N - 2, where N = number of block linked together. Any other numbers are illegal.

0000000000 (000h) = invalid setting

0000000001 (001h) = low watermark is 1 block

0000000010 (002h) = low watermark is 2 blocks

0111111111 (1FFh) = low watermark is 511 blocks

1111111111 (3FFh) = low watermark is 1023 blocks

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