MC68HC908AS32A Datasheet, PDF (175/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Description
Fast Data Tolerance
Figure 14-8 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.
RECEIVER
RT CLOCK
STOP
IDLE OR NEXT CHARACTER
DATA
SAMPLES
Figure 14-8. Fast Data
For an 8-bit character, data sampling of the stop bit takes the receiver
9 bit times × 16 RT cycles + 10 RT cycles = 154 RT cycles.
With the misaligned character shown in Figure 14-8, 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
-1---5---4-----^–----1----6---0-- ^{× 100 = 3.9·0%}
154
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 14-8, 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
1----7---0-----–_{-----1---7---6-- × 100 = 3.53%}
170
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:
• Address mark — An address mark is a 1 in the most significant bit 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 SCI 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.
MC68HC908AS32A Data Sheet, Rev. 2.0
Freescale Semiconductor
175