MC9S08RG60 Datasheet, PDF (151/232 Pages) Motorola, Inc – Microcontrollers

SoC Guide — MC9S08RG60/D Rev 1.10
When a program detects that the receive data register is full (RDRF = 1), it gets the data from the receive
data register by reading SCI1D. The RDRF flag is cleared by reading SCI1S1 while RDRF = 1 and then
reading SCI1D.
When polling is used, this sequence is naturally satisfied in the normal course of the user program. If
hardware interrupts are used, SCI1S1 must be read in the interrupt service routine (ISR). Normally, this is
done in the ISR anyway to check for receive errors, so the sequence is automatically satisfied.
The IDLE status flag includes logic that prevents it from getting set repeatedly when the RxD1 line
remains idle for an extended period of time. IDLE is cleared by reading SCI1S1 while IDLE = 1 and then
reading SCI1D. After IDLE has been cleared, it cannot become set again until the receiver has received at
least one new character and has set RDRF.
If the associated error was detected in the received character that caused RDRF to be set, the error flags
— noise flag (NF), framing error (FE), and parity error flag (PF) — get set at the same time as RDRF.
These flags are not set in overrun cases.
If RDRF was already set when a new character is ready to be transferred from the receive shifter to the
receive data buffer, the overrun (OR) flag gets set instead and the data and any associated NF, FE, or PF
condition is lost.
11.7 Additional SCI Functions
The following sections describe additional SCI functions.
11.7.1 8- and 9-Bit Data Modes
The SCI system (transmitter and receiver) can be configured to operate in 9-bit data mode by setting the
M control bit in SCI1C1. In 9-bit mode, there is a ninth data bit to the left of the MSB of the SCI data
register. For the transmit data buffer, this bit is stored in T8 in SCI1C3. For the receiver, the ninth bit is
held in R8 in SCI1C3.
For coherent writes to the transmit data buffer, write to the T8 bit before writing to SCI1D. For coherent
reads of the receive data buffer, read R8 before reading SCI1D because reading or writing SCI1D is the
final step in automatic clearing mechanisms for SCI flags.
If the bit value to be transmitted as the ninth bit of a new character is the same as for the previous character,
it is not necessary to write to T8 again. When data is transferred from the transmit data buffer to the
transmit shifter, the value in T8 is copied at the same time data is transferred from SCI1D to the shifter.
9-bit data mode typically is used in conjunction with parity to allow eight bits of data plus the parity in the
ninth bit. Or it is used with address-mark wakeup so the ninth data bit can serve as the wakeup bit. In
custom protocols, the ninth bit can also serve as a software-controlled marker.
Freescale Semiconductor
MC9S08RC/RD/RE/RG
151