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Functional Description
MC68HC908GZ60 MC68HC908GZ48 MC68HC908GZ32 Data Sheet, Rev. 5
Freescale Semiconductor
199
Fast Data Tolerance
Figure 14-9 shows how much a fast received character 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 there for the stop bit
data samples at RT8, RT9, and RT10.
Figure 14-9. Fast Data
For an 8-bit 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 14-9, 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 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 14-9, 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:
14.4.3.6 Receiver Wakeup
So that the MCU 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 SCC2 puts the
receiver into a standby state during which receiver interrupts are disabled.
Depending on the state of the WAKE bit in SCC1, either of two conditions on the RxD pin can bring the
receiver out of the standby state:
1.
Address mark — An address mark is a 1 in the MSB position of a received character. When the
WAKE bit is set, an address mark wakes the receiver from the standby state by clearing the RWU
bit. The address mark also sets the ESCI receiver full bit, SCRF. Software can then compare the
character containing the address mark to the user-defined address of the receiver. If they are the
same, the receiver remains awake and processes the characters that follow. If they are not the
same, software can set the RWU bit and put the receiver back into the standby state.
2.
Idle input line condition — When the WAKE bit is clear, an idle character on the RxD pin wakes the
receiver from the standby state by clearing the RWU bit. The idle character that wakes the receiver
IDLE OR NEXT CHARACTER
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%.
=