MC68HC908GR4MP datasheet(172/286 Pages) FREESCALE

172 / 286 page

Serial Communications Interface (SCI)

MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

172

Freescale Semiconductor

18.4.3.7 Fast Data Tolerance

Figure 18-8 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.

Figure 18-8. 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 18-8, 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

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 18-8, 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

18.4.3.8 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 PE1/RxD pin can bring

the receiver out of the standby state:

•

Address mark — An address mark is a logic 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

IDLE OR NEXT CHARACTER

STOP

DATA

SAMPLES

RECEIVER

RT CLOCK

154

160

–

154

--------------------------

100

3.90%

170

176

–

170

--------------------------

100

3.53%

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