AM79C970 datasheet(156/168 Pages) AMD

156 / 168 page

AMD

1-1023

Am79C970

Outline of the LAPP Flow

This section gives a suggested outline for a driver that

utilizes the LAPP feature of the PCnet-PCI controller.

Note: The labels in the following text are used as refer-

ences in the timeline diagram that follows.

SETUP:

The driver should set up descriptors in groups of 3, with

the OWN and STP bits of each set of three descriptors to

read as follows: 11b, 10b, 00b.

An option bit (LAPPEN) exists in CSR3, bit position 5;

the software should set this bit; When set, the LAPPEN

bit directs the PCnet-PCI controller to generate an IN-

TERRUPT when STP has been written to a receive de-

scriptor by the PCnet-PCI controller.

FLOW:

The PCnet-PCI controller polls the current receive de-

scriptor at some point in time before a message arrives.

The PCnet-PCI controller determines that this receive

buffer is OWNed by the PCnet-PCI controller and it

stores the descriptor information to be used when a

message does arrive.

N0:

Frame preamble appears on the wire, followed by

SFD and destination address.

N1:

The 64th byte of frame data arrives from the wire.

This causes the PCnet-PCI controller to begin

frame data DMA operations to the first buffer.

C0:

When the 64th byte of the message arrives, the

PCnet-PCI controller performs a lookahead op-

eration to the next receive descriptor. This de-

scriptor should be owned by the PCnet-PCI

controller .

C1:

The PCnet-PCI controller intermittently requests

the bus to transfer frame data to the first buffer as it

arrives on the wire.

S1:

The driver remains idle.

C2:

When the PCnet-PCI controller has completely

filled the first buffer, it writes status to the first

descriptor.

C3:

When the first descriptor for the frame has been

written, changing ownership from the PCnet-PCI

controller to the CPU, the PCnet-PCI controller

will generate an SRP INTERRUPT. (This interrupt

appears as a RINT interrupt in CSR0.)

S1:

The SRP INTERRUPT causes the CPU to switch

tasks to allow the PCnet-PCI controllers driver to

run.

C4:

During the CPU interrupt-generated task switch-

ing, the PCnet-PCI controller is performing a

lookahead operation to the third descriptor. At this

point in time, the third descriptor is owned by the

CPU.

Note: Even though the third buffer is not owned by

the PCnet-PCI controller, existing AMD Ethernet

controllers will continue to perform data DMA into

the buffer space that the controller already owns

(i.e., buffer number 2). The controller does not

know if buffer space in buffer number 2 will be suf-

ficient or not, for this frame, but it has no way to tell

except by trying to move the entire message into

that space. Only when the message does not fit

will it signal a buffer error condition – there is no

need to panic at the point that it discovers that it

does not yet own descriptor number 3.

S2:

The first task of the drivers interrupt service rou-

tine is to collect the header information from the

PCnet-PCI controllers first buffer and pass it to the

application.

S3:

The application will return an application buffer

pointer to the driver. The driver will add an offset to

the

application data buffer pointer, since the

PCnet-PCI controller will be placing the first por-

tion of the message into the first and second buff-

ers. (The modified application data buffer pointer

will only be directly used by the PCnet-PCI con-

troller when it reaches the third buffer.) The driver

will place the modified data buffer pointer into the

final descriptor of the group (#3) and will grant

ownership of this descriptor to the PCnet-PCI

controller.

C5:

Interleaved with S2, S3 and S4 driver activity, the

PCnet-PCI controller will write frame data to buffer

number 2.

S4:

The driver will next proceed to copy the contents

of the PCnet-PCI controllers first buffer to the be-

ginning of the application space. This copy will be

to the exact (unmodified) buffer pointer that was

passed by the application.

S5:

After copying all of the data from the first buffer

into the beginning of the application data buffer,

the driver will begin to poll the ownership bit of the

second descriptor. The driver is waiting for the

PCnet-PCI controller to finish filling the second

buffer.

C6:

At this point, knowing that it had not previously

owned the third descriptor, and knowing that the

current message has not ended (there is more

data in the FIFO), the PCnet-PCI controller will

make a last ditch lookahead to the final (third) de-

scriptor. This time, the ownership will be TRUE

(i.e. the descriptor belongs to the controller), be-

cause the driver wrote the application pointer into

this descriptor and then changed the ownership to

give the descriptor to the PCnet-PCI controller

back at S3. Note that if steps S1, S2 and S3 have

not completed at this time, a BUFF error will result.

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