MC68HC05B6_13 Datasheet, PDF (77/302 Pages) Freescale Semiconductor, Inc – Microcontrollers

6.5
Data format
Receive data or transmit data is the serial data that is transferred to the internal data bus from the
receive data input pin (RDI) or from the internal bus to the transmit data output pin (TDO). The
non-return-to-zero (NRZ) data format shown in Figure 6-3 is used and must meet the following
criteria:
– The idle line is brought to a logic one state prior to transmission/reception of
a character.
– A start bit (logic zero) is used to indicate the start of a frame.
– The data is transmitted and received least significant bit first.
– A stop bit (logic one) is used to indicate the end of a frame. A frame consists
of a start bit, a character of eight or nine data bits, and a stop bit.
– A break is defined as the transmission or reception of a low (logic zero) for at
least one complete frame time (10 zeros for 8-bit format, 11 zeros for 9-bit).
6
Control bit M selects
8 or 9 bit data
Idle line
012345678
0
Start
Stop Start
Figure 6-3 Data format
6.6
Receiver wake-up operation
The receiver logic hardware also supports a receiver wake-up function which is intended for
systems having more than one receiver. With this function a transmitting device directs messages
to an individual receiver or group of receivers by passing addressing information as the initial
byte(s) of each message. The wake-up function allows receivers not addressed to remain in a
dormant state for the remainder of the unwanted message. This eliminates any further software
overhead to service the remaining characters of the unwanted message and thus improves
system performance.
The receiver is placed in wake-up mode by setting the receiver wake-up bit (RWU) in the SCCR2
register. While RWU is set, all of the receiver related status flags (RDRF, IDLE, OR, NF, and FE)
are inhibited (cannot become set). Note that the idle line detect function is inhibited while the RWU
bit is set. Although RWU may be cleared by a software write to SCCR2, it would be unusual to do
so. Normally RWU is set by software and is cleared automatically in hardware by one of the two
methods described below.
MC68HC05B6
Rev. 4.1
SERIAL COMMUNICATIONS INTERFACE
Freescale
6-5