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| 型號： | QSMRM |
| 英文描述： | QSMRM QSM Queued Serial Module Reference Manual |
| 中文描述： | QSMRM QSM排隊(duì)串行模塊參考手冊 |
| 文件頁數(shù)： | 78/112頁 |
| 文件大小： | 1496K |
| 代理商： | QSMRM |
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MOTOROLA
5-22
SCI SUBMODULE
QSM
REFERENCE MANUAL
5.4.2.2 Receiver Wakeup
The SCI receiver hardware provides a receiver wakeup function to support multinode
networks containing more than one receiver. This function allows the transmitting de-
vice to direct a message to an individual receiver or group of receivers by sending an
address frame at the start of a message. All receivers not addressed for the current
message invoke the receiver wakeup function, which effectively allows them to sleep
through the rest of the message. Therefore, the CPU is alleviated from servicing reg-
ister RDR, resulting in increased system performance.
The SCI receiver is placed in wakeup mode by writing a one to RWU in SCCR1. While
RWU is set, all receiver status flag bits are inhibited from being set. Note that the IDLE
flag cannot be used when RWU is set. Although the CPU can clear RWU by writing a
zero to SCCR1, it is normally left alone by software and is cleared automatically by
hardware in one of two methods: idle-line wakeup or address-mark wakeup.
WAKE in SCCR1 determines which method of wakeup is to be employed. If WAKE =
0, idle-line wakeup is selected. This method is compatible with the method originally
used on the MC6801. If WAKE = 1, address-mark wakeup is selected, which uses a
one in the MSB of data to denote an address frame and uses a zero to denote a normal
data frame. Each method has its particular advantages and disadvantages.
Both wakeup methods require a software device addressing and recognition scheme
and, therefore, can conform to all transmitters and receivers. The addressing informa-
tion is usually the first frame(s) of the message. Receivers for which the message is
not intended may set RWU and go back to sleep for the remainder of the message.
Idle-line wakeup allows a receiver to sleep until an idle line is detected, causing RWU
to be cleared by the receiver and causing the receiver to wake up. The receiver waits
through the idle times for the first frame of the next message. If the receiver is not the
intended addressee, RWU may be set to put the receiver back to sleep. This method
of receiver wakeup requires that a minimum of one frame of idle line be imposed be-
tween messages. As previously stated, no idle time is allowed between frames within
a message.
Address-mark wakeup uses a special frame format to wake up the receiver. All frames
consist of seven (or eight) data bits plus an MSB that indicates an address frame when
set to a one. The first frame of each message should be an address frame. All receiv-
ers in the system must use a software scheme to determine which messages address
them. If the message is not intended for a particular receiver, the CPU sets RWU so
that the receiver goes back to sleep, thereby eliminating additional CPU overhead for
servicing the rest of the message.
When the first frame of a new message is received with the MSB set, denoting an ad-
dress frame, RWU is cleared. The byte is received normally, transferred to register
RDR, and the RDRF flag is set. Address-mark wakeup allows messages to include idle
times between frames and eliminates idle time between messages; however, an effi-
ciency loss results from the extra bit-time (address bit) that is required on all frames.
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
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