MC68HC05B6_13 Datasheet, PDF (78/302 Pages) Freescale Semiconductor, Inc – Microcontrollers

6.6.1 Idle line wake-up
In idle line wake-up mode, a dormant receiver wakes up as soon as the RDI line becomes idle.
Idle is defined as a continuous logic high level on the RDI line for ten (or eleven) full bit times.
Systems using this type of wake-up must provide at least one character time of idle between
messages to wake up sleeping receivers, but must not allow any idle time between characters
within a message.
6.6.2 Address mark wake-up
In address mark wake-up, the most significant bit (MSB) in a character is used to indicate whether
it is an address (1) or data (0) character. Sleeping receivers will wake up whenever an address
character is received. Systems using this method for wake-up would set the MSB of the first
6
character of each message and leave it clear for all other characters in the message. Idle periods
may be present within messages and no idle time is required between messages for this wake-up
method.
6.7
Receive data in (RDI)
Receive data is the serial data that is applied through the input line and the SCI to the internal bus.
The receiver circuitry clocks the input at a rate equal to 16 times the baud rate. This time is referred
to as the RT rate in Figure 6-4 and as the receiver clock in Figure 6-2.
The receiver clock generator is controlled by the baud rate register, as shown in Figure 6-1 and
Figure 6-2; however, the SCI is synchronized by the start bit, independent of the transmitter.
Once a valid start bit is detected, the start bit, each data bit and the stop bit are sampled three
times at RT intervals 8 RT, 9 RT and 10 RT (1 RT is the position where the bit is expected to start),
as shown in Figure 6-5. The value of the bit is determined by voting logic which takes the value of
the majority of the samples. A noise flag is set when all three samples on a valid start bit or data
bit or the stop bit do not agree.
6.8
Start bit detection
When the input (idle) line is detected low, it is tested for three more sample times (referred to as
the start edge verification samples in Figure 6-4). If at least two of these three verification samples
detect a logic zero, a valid start bit has been detected, otherwise the line is assumed to be idle. A
noise flag is set if one of the three verification samples detect a logic one, thus a valid start bit
could be assumed with a set noise flag present.
Freescale
6-6
SERIAL COMMUNICATIONS INTERFACE
MC68HC05B6
Rev. 4.1