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Chapter 11 Serial Communication Interface (S12SCIV5)
MC9S12XDP512 Data Sheet, Rev. 2.17
Freescale Semiconductor
509
11.4.6.5.2
Fast Data Tolerance
Figure 11-29
shows how much a fast received frame can be misaligned. The fast stop bit ends at RT10
instead of RT16 but is still sampled at RT8, RT9, and RT10.
Figure 11-29. Fast Data
Foran8-bitdatacharacter,ittakesthereceiver9bittimesx16RTrcycles+10RTrcycles=154RTrcycles
to finish data sampling of the stop bit.
Withthemisalignedcharactershownin
Figure 11-29
,thereceivercounts154RTrcyclesatthepointwhen
the count of the transmitting device is 10 bit times x 16 RTt cycles = 160 RTt cycles.
The maximum percent difference between the receiver count and the transmitter count of a fast 8-bit
character with no errors is:
((160 – 154) / 160) x 100 = 3.75%
Fora9-bitdatacharacter,ittakesthereceiver10bittimesx16RTrcycles+10RTrcycles=170RTrcycles
to finish data sampling of the stop bit.
Withthemisalignedcharactershownin
Figure 11-29
,thereceivercounts170RTrcyclesatthepointwhen
the count of the transmitting device is 11 bit times x 16 RTt cycles = 176 RTt cycles.
The maximum percent difference between the receiver count and the transmitter count of a fast 9-bit
character with no errors is:
((176 – 170) /176) x 100 = 3.40%
11.4.6.6
Receiver Wakeup
To enable the SCI to ignore transmissions intended only for other receivers in multiple-receiver systems,
thereceivercanbeputintoastandbystate.Settingthereceiverwakeupbit,RWU,inSCIcontrolregister 2
(SCICR2) puts the receiver into standby state during which receiver interrupts are disabled.The SCI will
still load the receive data into the SCIDRH/L registers, but it will not set the RDRF flag.
Thetransmittingdevicecanaddressmessagestoselectedreceiversbyincludingaddressinginformationin
the initial frame or frames of each message.
The WAKE bit in SCI control register 1 (SCICR1) determines how the SCI is brought out of the standby
state to process an incoming message. The WAKE bit enables either idle line wakeup or address mark
wakeup.
Idle or Next Frame
Stop
R
R
R
R
R
R
R
R
R
Data
Samples
R
R
R
R
R
R
R
Receiver
RT Clock