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When submitted, the stack issues a command to de ne the block length as aBlkLen bytes for the subsequent block write command. It then issues a write multiple block command to continually transfer blocks from the host to the card, starting at address aMemoryP in system memory, and offset aDevAddr on the card. Once aLength bytes have been transferred, the stack issues a stop command to terminate the transfer. Engage() is used to enque the session for execution on the DMMCStack object once it has been con gured. ResponseP() returns a pointer to a buffer containing the last command response received by the session. The controller is designed to accept more than one client request on a stack at any given time. This could happen on multi-card stacks, or on single card stacks containing multi-function cards where multiple drivers have session engaged simultaneously. The controller attempts to manage the sessions as ef ciently as it can, by internally scheduling them onto the bus. When the current session becomes blocked waiting on an event, the controller will attempt to reschedule another session in its place.
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Bus con guration and error recovery
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Referring still to Figure 13.12, the class TMmcStackConfig is used to hold bus con guration settings for a stack. These settings are such things as the bus clock rate, whether to try re-issuing commands on error, how long to wait for a response from the card and so on. The stack owns an instance of this class (not shown on the diagram) containing the default settings that are normally applied. Each session also owns an instance of this class, the member iConfig, which normally contains a copy of the defaults. However, if it chooses, the client may over-ride the con guration settings for any bus operation it submits by altering the contents of iConfig. These changes only remain in effect for the period that the session remains current. The controller is normally con gured to automatically retry failed operations when any of the following errors are detected: Timeout waiting for a command response from a card A CRC error is detected in a response A timeout waiting for data transfer to commence during a data read or write command A CRC error detected in a data block during data transfer. For certain other errors, such as if the card state is found to be inconsistent with the command being issued, the controller will attempt to recover by re-initializing the entire stack before retrying the failed operation.
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13.5.4 Card power handling
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When the controller detects a door-open event, it tries to remove power from the card as soon as possible. It does not remove power immediately if a bus operation is in progress, because it wouldn t be a good idea to remove power from a card in the middle of writing a block, as this could corrupt the block. In this case, power-down is deferred until the end of the MultiMediaCard session. Attempts to engage a new session while the door is open will fail immediately though. So, to avoid the situation in which a card is physically unplugged while a command is still completing, driver requests have to be kept short enough to ensure that they can always be completed in the time between the door open event and the time the card is physically removed. This means that long multi-block write commands have to be avoided, despite the improved rate of data transfer they provide over shorter block transfers. It is very important that the phone provides a door mechanism and circuitry that gives early warning of potential card removal. The controller is normally con gured to implement a bus inactivity power-down scheme to save power. If the inactivity period elapses, then the controller automatically removes power from the cards. The length of this inactivity timeout period is set by the particular mobile phone. As I said in Section 13.3.3.5, the local media sub-system does not initialize removable media devices as soon as they are inserted, but instead waits until the rst request on the drive. Nevertheless, this request generally arrives almost immediately after card insertion, because applications receive noti cation of the disk insertion event from the le server and then interrogate the new card. For MultiMediaCards, initialization involves applying bus power and then performing the card identi cation process. This entails issuing a series of broadcast and addressed commands over the bus, and is handled asynchronously by the controller. (All requests on the stack that involve bus activity are inherently long running operations that have to be handled asynchronously.) Initialization proceeds as follows. First, the cards in the stack are reset, and then their operating voltage range is ascertained to ensure this is compatible with that of the host phone. The host reads the 128-bit unique ID that identi es each card. It then allocates each card a shorter Relative Card Address (RCA), which is used thereafter to address that card. Finally, the host reads back data from the card concerning its operating characteristics, to check that these are compatible with the host. Now the card is available for data transfer. This entire process is carried out in the rst phase of drive mounting before any media drivers are opened. I/O drivers don t use the local media sub-system, and so they need to ensure that the bus is powered and the stack is initialized when they open. However, once an I/O driver has opened successfully, it doesn t need to bother about the card subsequently becoming powered down
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