Functional Description
MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor
165
Fast Data Tolerance
Figure 14-14 shows how much a fast received frame can be misaligned without causing a noise error or
a framing error. The fast stop bit ends at RT10 instead of RT16 but is still sampled at RT8, RT9, and RT10.
Figure 14-14. Fast Data
For an 8-bit data character, data sampling of the stop bit takes the receiver
9bit times
× 16 RT cycles + 10 RT cycles = 154 RT cycles.
With the misaligned character shown in
Figure 14-14, the receiver counts 154 RT cycles at the point when
the count of the transmitting device is 10 bit times
× 16 RT cycles = 160 RT cycles.
The maximum percent difference between the receiver count and the transmitter count of a fast 8-bit
character with no errors is:
For a 9-bit data character, data sampling of the stop bit takes the receiver
10 bit times
× 16 RT cycles + 10 RT cycles = 170 RT cycles.
With the misaligned character shown in
Figure 14-14, the receiver counts 170 RT cycles at the point when
the count of the transmitting device is: 11 bit times
× 16 RT cycles = 176 RT cycles.
The maximum percent difference between the receiver count and the transmitter count of a fast 9-bit
character with no errors is:
14.5.4.6 Receiver Wakeup
So that the SCI can ignore transmissions intended only for other receivers in multiple-receiver systems,
the receiver can be put into a standby state. Setting the receiver wakeup bit, RWU, in SCI control register
2 (SCCR2) puts the receiver into a standby state during which receiver interrupts are disabled.
The transmitting device can address messages to selected receivers by including addressing information
in the initial frame or frames of each message.
The WAKE bit in SCI control register 1 (SCCR1) 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 input line wakeup (WAKE = 0) — In this wakeup method, an idle condition on the RXD pin
clears the RWU bit and wakes up the SCI. The initial frame or frames of every message contain
addressing information. All receivers evaluate the addressing information, and receivers for which
IDLE OR NEXT FRAME
STOP
RT1
RT2
RT3
RT4
RT5
RT6
RT7
RT8
RT9
RT10
RT11
RT12
RT13
RT14
RT15
RT16
DATA
SAMPLES
RECEIVER
RT CLOCK
154
160
–
154
--------------------------
100
×
3.90%
=
170
176
–
170
--------------------------
100
×
3.53%
=