HCS08 datasheet(180/290 Pages) MOTOROLA

180 / 290 page

Serial Communications Interface (SCI) Module

MC9S08GB/GT

180

MOTOROLA

When a program detects that the receive data register is full (RDRF = 1), it gets the data from the receive

data register by reading SCIxD. The RDRF flag is cleared by reading SCIxS1 while RDRF = 1 and then

reading SCIxD.

When polling is used, this sequence is naturally satisfied in the normal course of the user program. If

hardware interrupts are used, SCIxS1 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 which prevents it from getting set repeatedly when the RxD line

remains idle for an extended period of time. IDLE is cleared by reading SCIxS1 while IDLE = 1 and then

reading SCIxD. Once 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.

11.8 Additional SCI Functions

The following sections describe additional SCI functions.

11.8.1 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 SCIxC1. In 9-bit mode, there is a ninth data bit to the left of the MSB of the SCI data

held in R8 in SCIxC3.

For coherent writes to the transmit data buffer, write to the T8 bit before writing to SCIxD. For coherent

reads of the receive data buffer, read R8 before reading SCIxD because reading or writing SCIxD 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 SCIxD to the shifter.

Nine-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.

HCS08 bảng dữ liệu(PDF) 180 Page - Motorola, Inc