MM912F634CV2AP datasheet(118/335 Pages) FREESCALE

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Functional Description and Application Information

Serial Communication Interface (S08SCIV4)

MM912F634

Freescale Semiconductor

118

4.15.3.3.2.1

Idle-line Wake-up

When WAKE = 0, the receiver is configured for idle-line wake-up. In this mode, RWU is cleared automatically when the receiver

detects a full character time of the idle-line level. The M control bit selects 8-bit or 9-bit data mode that determines how many bit

times of idle are needed to constitute a full character time (10 or 11 bit times because of the start and stop bits).

When RWU is one and RWUID is zero, the idle condition that wakes up the receiver does not set the IDLE flag. The receiver

wakes up and waits for the first data character of the next message which will set the RDRF flag and generate an interrupt if

enabled. When RWUID is one, any idle condition sets the IDLE flag and generates an interrupt if enabled, regardless of whether

RWU is zero or one.

The idle-line type (ILT) control bit selects one of two ways to detect an idle line. When ILT = 0, the idle bit counter starts after the

start bit so the stop bit and any logic 1s at the end of a character count toward the full character time of idle. When ILT = 1, the

idle bit counter does not start until after a stop bit time, so the idle detection is not affected by the data in the last character of the

previous message.

4.15.3.3.2.2

Address-Mark Wake-up

When WAKE = 1, the receiver is configured for address-mark wake-up. In this mode, RWU is cleared automatically when the

receiver detects a logic 1 in the most significant bit of a received character (eighth bit in M = 0 mode and ninth bit in M = 1 mode).

Address-mark wake-up allows messages to contain idle characters but requires that the MSB be reserved for use in address

frames. The logic 1 MSB of an address frame clears the RWU bit before the stop bit is received and sets the RDRF flag. In this

case the character with the MSB set is received even though the receiver was sleeping during most of this character time.

4.15.3.4

Interrupts and Status Flags

The SCI system has three separate interrupt vectors to reduce the amount of software needed to isolate the cause of the

interrupt. One interrupt vector is associated with the transmitter for TDRE and TC events. Another interrupt vector is associated

with the receiver for RDRF, IDLE, RXEDGIF and LBKDIF events, and a third vector is used for OR, NF, FE, and PF error

conditions. Each of these ten interrupt sources can be separately masked by local interrupt enable masks. The flags can still be

polled by software when the local masks are cleared to disable generation of hardware interrupt requests.

The SCI transmitter has two status flags that optionally can generate hardware interrupt requests. Transmit data register empty

(TDRE) indicates when there is room in the transmit data buffer to write another transmit character to SCID. If the transmit

interrupt enable (TIE) bit is set, a hardware interrupt will be requested whenever TDRE = 1. Transmit complete (TC) indicates

that the transmitter is finished transmitting all data, preamble, and break characters and is idle with TxD at the inactive level. This

flag is often used in systems with modems to determine when it is safe to turn off the modem. If the transmit complete interrupt

enable (TCIE) bit is set, a hardware interrupt will be requested whenever TC = 1. Instead of hardware interrupts, software polling

may be used to monitor the TDRE and TC status flags if the corresponding TIE or TCIE local interrupt masks are 0s.

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

SCID. The RDRF flag is cleared by reading SCIS1 while RDRF = 1 and then reading SCID.

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

used, SCIS1 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 RxD line remains idle for an extended

period of time. IDLE is cleared by reading SCIS1 while IDLE = 1 and then reading SCID. 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 the data along with any associated NF, FE, or PF condition is lost.

MM912F634CV2AP Datasheet(PDF) 118 Page - Freescale Semiconductor, Inc

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