AM79C960 datasheet(35/127 Pages) AMD

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P R E L I M I N A R Y

AMD

1-377

Am79C960

A typical receive poll occurs under the following

conditions:

1) PCnet-ISA controller does not possess ownership

of the current RDTE and

the poll time has elapsed and

RXON = 1,

2) PCnet-ISA controller does not possess ownership

of the next RDTE and

the poll time has elapsed and

RXON = 1.

If RXON = 0, the PCnet-ISA controller will never poll

RDTE locations.

If RXON=1, the system should always have at least one

RDTE available for the possibility of a receive event.

When there is only one RDTE, there is no polling for next

RDTE.

A typical transmit poll occurs under the following

conditions:

1) PCnet-ISA controller does not possess ownership

of the current TDTE and

DPOLL = 0 and

TXON = 1 and

the poll time has elapsed,

2) PCnet-ISA controller does not possess ownership

of the current TDTE and

DPOLL = 0 and

TXON = 1 and

a packet has just been received,

3) PCnet-ISA controller does not possess ownership

of the current TDTE and

DPOLL = 0 and

TXON = 1 and

a packet has just been transmitted.

The poll time interval is nominally defined as 32,768

crystal clock periods, or 1.6 ms. However, the poll time

other microcode controlled operation will interrupt the

incrementing of the poll count register. For example,

when a receive packet is accepted by the PCnet-ISA

controller, the device suspends execution of the poll-

time-incrementing

microcode

that

receive

microcode routine may instead be executed. Poll-time-

incrementing code is resumed when the receive

operation has completely finished. Note, however, that

following the completion of any receive or transmit op-

eration, a poll operation will always be performed. The

poll time count register is never reset. Note that if a non-

default value is desired, then a strict sequence of setting

the INIT bit in CSR0, waiting for INITDONE, then writing

to CSR47, and then setting STRT in CSR0 must be ob-

served, otherwise the default value will not be

overwritten. See the CSR47 section for details.

Setting the TDMD bit of CSR0 will cause the microcode

controller to exit the poll counting code and immediately

perform a polling operation. If RDTE ownership has not

been previously established, then an RDTE poll will be

performed ahead of the TDTE poll.

Transmit Descriptor Table Entry (TDTE)

If, after a TDTE access, the PCnet-ISA controller finds

that the OWN bit of that TDTE is not set, then the

PCnet-ISA controller resumes the poll time count and

reexamines the same TDTE at the next expiration of the

poll time count.

If the OWN bit of the TDTE is set, but STP = 0, the

PCnet-ISA controller will immediately request the bus in

order to reset the OWN bit of this descriptor; this condi-

tion would normally be found following a LCOL or

RETRY error that occurred in the middle of a transmit

packet chain of buffers. After resetting the OWN bit of

this descriptor, the PCnet-ISA controller will again im-

mediately request the bus in order to access the next

TDTE location in the ring.

If the OWN bit is set and the buffer length is 0, the OWN

bit will be reset. In the LANCE the buffer length of 0 is

interpreted as a 4096-byte buffer. It is acceptable to

have a 0 length buffer on transmit with STP = 1 or STP =

and ENP = 1. It is not acceptable to have 0 length

buffer with STP = 0

and ENP = 1.

If the OWN bit is set and the start of packet (STP) bit is

set, then microcode control proceeds to a routine that

will enable transmit data transfers to the FIFO.

If the transmit buffers are data chained (ENP=0 in the

first buffer), then the PCnet-ISA controller will look

ahead to the next transmit descriptor after it has per-

formed at least one transmit data transfer from the first

buffer. More than one transmit data transfer may possi-

bly take place, depending upon the state of the

transmitter. The transmit descriptor lookahead reads

TMD0 first and TMD1 second. The contents of TMD0

and TMD1 will be stored in Next TX Descriptor Address

(CSR32), Next TX Byte Count (CSR66) and Next TX

Status (CSR67) regardless of the state of the OWN bit.

This transmit descriptor lookahead operation is per-

formed only once.

If the PCnet-ISA controller does not own the next TDTE

(i.e. the second TDTE for this packet), then it will com-

plete transmission of the current buffer and then update

the status of the current (first) TDTE with the BUFF and

UFLO bits being set. This will cause the transmitter to be

disabled (CSR0, TXON = 0). The PCnet-ISA controller

will have to be restarted to restore the transmit function.

The situation that matches this description implies that

the system has not been able to stay ahead of the

PCnet-ISA controller in the transmit descriptor ring and,

therefore, the condition is treated as a fatal error. To

avoid this situation, the system should always set the

transmit chain descriptor own bits in reverse order.

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