DS31256 datasheet(144/183 Pages) MAXIM

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DS31256 256-Channel, High-Throughput HDLC Controller

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10.2.4

Status Bits (PCMD1)

The upper word in the PCMD1 register is the status portion, which reports events as they occur. As mentioned

earlier, reads of the status portion occur normally, but writes are unique in that bits can only be reset (i.e., forced to

0). This occurs when a 1 is written to a bit position. Writes with a 0 to a bit position have no effect. This allows

individual bits to be reset.

Bits 16 to 20/Reserved. These read-only bits are forced to 0 by the device.

Bit 21/66MHz Capable (66MHz). This read-only bit is forced to 0 by the device to indicate that it is not capable

of running at 66MHz.

Bit 22/User-Definable Features Capable (UDF). This read-only bit is forced to 0 by the device to indicate that it

does not support user-definable features.

Bit 23/Fast Back-to-Back Capable Target (FBBCT). This read-only bit is forced to 1 by the device to indicate

that it is capable of accepting fast back-to-back transactions when the transactions are not from the same agent.

Bit 24/PCI Parity Error Reported (PARR). This read-only bit is forced to 0 by the device since the device

cannot act as a bus master.

Bits 25, 26/Device Timing Select Bits 0 and 1 (DTS0 and DTS1). These read-only bits are forced to 01b by the

device to indicate that they are capable of the medium timing requirements for the

PDEVSEL signal.

Bit 27/Target Abort Initiated (TABT). This read/write bit is set to 1 when the device terminates a bus

transaction with a target abort. This occurs only when the local bus is operating in the bus arbitration mode and the

local bus does not have bus control when the host requests access. This bit can be reset (set to 0) by the host by

writing a 1 to this bit.

Bit 28/Target Abort Detected by Master (TABTM). This read-only bit is forced to 0 by the device since the

device cannot act as a bus master.

Bit 29/Master Abort (MABT). This read-only bit is forced to 0 by the device since the device cannot act as a bus

master.

Bit 30/PCI System Error Reported (PSE). This read/write bit is set to 1 when the device asserts the

PSERR

signal (even if it is disabled through the PSEC command bit). This bit can be reset (set to 0) by the host by writing

1 to this bit.

Bit 31/PCI Parity Error Reported (PPE). This read/write bit is set to 1 when the device detects a parity error.

The host can reset this bit (set to 0) by writing a 1 to this bit.

DS31256 数据表(PDF) 144 Page - Maxim Integrated Products