CS4210 datasheet(21/102 Pages) NSC

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Operational Description (Continued)

3.3.2.1

ContextControl.run

The ContextControl.run bit is set by software when the

CS4210 is to begin processing descriptors for the context.

Before software sets ContextControl.run, ContextCon-

trol.active must not be set, and the CommandPtr register

for the context must contain a valid descriptor block

address and a Z value that is appropriate for the descriptor

block address. Software may stop the CS4210 from further

processing of a context by clearing ContextControl.run.

When a ContextControl.run is cleared, the CS4210 will

stop processing of the context in a manner that will not

impact the operation of any other context or DMA control-

ler. The CS4210 may require a significant amount of time

to safely stop processing for a context but when the

CS4210 does stop, it clears ContextControl.active. If soft-

ware clears a ContextControl.run for an isochronous con-

text while the CS4210 is processing a packet for the

context, the CS4210 continues to receive or transmit the

packet and update descriptor status. The CS4210 does,

however, stop at the conclusion of that packet. If Context-

Control.run is cleared for a non-isochronous context, the

CS4210 may stop processing at any convenient point as

long as the context and descriptors end up in a consistent

state (e.g., status updated if a packet was sent and

acknowledged). Clearing ContextControl.run may have

other side effects that are DMA controller dependent.

These effects are described in the subsections of Section

4.4 "OHCI Configuration Registers" starting on page 48

that cover each of the DMA controllers. When software

clears ContextControl.run and the CS4210 has stopped,

the CS4210 is not necessarily in a state that can be

restarted simply by setting ContextControl.run. Software

should always ensure that CommandPtr.descriptorAddress

and CommandPtr.Z are set to valid values before setting

ContextControl.run.

3.3.2.2

ContextControl.wake

When software adds to a list of descriptors for a context,

the CS4210 may have already read the descriptor that was

at the end of the list before it was updated. The value that

the CS4210 read may contain a Z value of zero indicating

the end of the descriptor list. The ContextControl.wake bit

provides a simple semaphore to the hardware to indicate

that the list may have changed since the last time the

CS4210 read a descriptor. Therefore, if the CS4210 had

fetched a descriptor and the indicated branch address had

a Z value of zero, then the CS4210 rereads the pointer

value.

For transmit contexts and receive contexts in buffer-fill

mode (a mode in which a context can receive multiple

packets into one data buffer), if the Z value is still zero, then

the end of the list was reached and the CS4210 clears

ContextControl.active. For receive contexts in buffer-fill

mode, if the Z value is still zero on the reread, then the

packet cannot be accepted.

For asynchronous contexts, the CS4210 returns the appro-

priate ack_busy* code. In addition, the CS4210 “backs out”

the packet by not updating the buffer’s byte count (res-

Count), and flushes the packet from the FIFO. The CS4210

does not go inactive, as there is still buffer space available

and software is attempting to provide more buffer space.

For both transmit and receive contexts, if the Z value is now

non-zero, the CS4210 continues processing. In order to

ensure that a wake condition is not missed, the CS4210

clears ContextControl.wake before it reads or rereads a

descriptor. ContextControl.wake is ignored when Context-

Control.run is zero.

3.3.2.3

ContextControl.active

ContextControl.active is set and cleared only by the

CS4210. It is set when the CS4210 receives an indication

from software that a valid descriptor is available for pro-

cessing. This indication occurs as a result of software set-

ting

the

ContextControl.run

software

setting

ContextControl.wake

while

ContextControl.run

set.

Thereare four cases inwhich theCS4210clearsContext-

Control.active:

When a branch is indicated by a descriptor but the Z

value of the branch address is 0.

When software clears ContextControl.run and the

CS4210 has reached a safe stopping point.

While ContextControl.dead is set.

After a hardware or software reset of the CS4210.

Additionally,

for

the

asynchronous

transmit

contexts

(request and response), the CS4210 clears ContextCon-

trol.active when a bus reset occurs. When ContextCon-

trol.active is cleared and ContextControl.run is already

clear, the CS4210 sets the IntEvent bit for the context. This

interrupt is the same interrupt that would have been gener-

ated by the context if a completed descriptor had indicated

that an interrupt should be generated.

3.3.2.4

ContextControl.dead

ContextControl.dead is used to indicate a fatal error in pro-

cessing a descriptor. When ContextControl.dead is set by

the CS4210, ContextControl.active is immediately cleared

but ContextControl.run remains set. In addition, setting

ContextControl.dead causes an unrecoverableError inter-

rupt event and blocks a normal context event interrupt from

being set. ContextControl.dead is immediately cleared

when software clears ContextControl.run or by either a

hardware or software reset of the CS4210. Software can

determine the cause of a context going dead by checking

the ContextControl.event code. The defined reasons for the

CS4210 to set ContextControl.dead are described in Sec-

tion 3.7 "Host Bus Errors" on page 28.

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