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Universal Serial Bus Interface
MCF5253 Reference Manual, Rev. 1
Freescale Semiconductor
24-83
Figure 24-49. Generic Queue Head Unlink Scenario
Alternatively, a host controller implementation is allowed to traverse the entire asynchronous schedule list
(for example, observed the head of the queue (twice)) before setting the Advance on Async status bit.
The software may re-use the memory associated with the removed queue heads after it observes the
Interrupt on Async Advance status bit is set, following assertion of the doorbell. The software should
acknowledge the Interrupt on Async Advance status as indicated in the USBSTS register, before using the
doorbell handshake again
24.9.9.3
Empty Asynchronous Schedule Detection
EHCI uses two bits to detect when the asynchronous schedule is empty. The queue head data structure (see
) defines an H-bit in the queue head, which allows the software to mark a queue head as being
the head of the reclaim list. host controller also keeps a 1-bit flag in the USBSTS register (Reclamation)
that is cleared when the host controller observes a queue head with the H-bit set. The reclamation flag in
the status register is set when any USB transaction from the asynchronous schedule is executed (or
whenever the asynchronous schedule starts, see
Section 24.9.9.4, “Asynchronous Schedule Traversal:
If the controller ever encounters an H-bit of one and a Reclamation bit of zero, the controller simply stops
traversal of the asynchronous schedule.
An example illustrating the H-bit in a schedule is shown in
Memory State
B
A
C
D
A
USBCMD Interrupt on
Async-Advance Doorbell = 0
Before Unlink
HC State
Memory State
B
A
D
A
USBCMD Interrupt on
Async-Advance Doorbell = 1
After Unlink (B, C) and at Doorbell
HC State
C
USBSTS Interrupt on Async-Advance = 0
Memory State
B
A
D
D
USBCMD Interrupt on
Async-Advance Doorbell = 0
After Doorbell
HC State
C
USBSTS Interrupt on Async-Advance = 1
Содержание MCF5253
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