18F8680 datasheet(244/496 Pages) MICROCHIP

244 / 496 page

PIC18F6585/8585/6680/8680

DS30491C-page 242

 2004 Microchip Technology Inc.

18.2.4

AUTO-WAKE-UP ON SYNC BREAK

CHARACTER

During Sleep mode, all clocks to the USART are

suspended. Because of this, the Baud Rate Generator

is inactive and a proper byte reception cannot be per-

formed. The auto-wake-up feature allows the controller

to wake-up due to activity on the RX/DT line while the

USART is operating in Asynchronous mode.

The auto-wake-up feature is enabled by setting the

WUE bit (BAUDCON<1>). Once set, the typical receive

sequence on RX/DT is disabled and the USART

remains in an Idle state monitoring for a wake-up event

independent of the CPU mode. A wake-up event con-

sists of a high-to-low transition on the RX/DT line. (This

coincides with the start of a sync break or a wake-up

signal character for the LIN protocol.)

Following a wake-up event, the module generates an

RCIF interrupt. The interrupt is generated synchro-

nously to the Q clocks in normal operating modes

(Figure 18-7) and asynchronously, if the device is in

Sleep mode (Figure 18-8). The interrupt condition is

cleared by reading the RCREG register.

The WUE bit is automatically cleared once a low-to-

high transition is observed on the RX line following the

wake-up event. At this point, the USART module is in

Idle mode and returns to normal operation. This signals

to the user that the sync break event is over.

18.2.4.1

Special Considerations Using

Auto-Wake-up

Since auto-wake-up functions by sensing rising edge

transitions on RX/DT, information with any state changes

before the Stop bit may signal a false end-of-character

and cause data or framing errors. To work properly,

therefore, the initial character in the transmission must

be all ‘0’s. This can be 00h (8 bytes) for standard RS-232

devices or 000h (12 bits) for LIN bus.

Oscillator start-up time must also be considered,

especially in applications using oscillators with longer

start-up intervals (i.e., XT or HS mode). The sync break

(or wake-up signal) character must be of sufficient

length and be followed by a sufficient interval to allow

enough time for the selected oscillator to start and

provide proper initialization of the USART.

18.2.4.2

Special Considerations Using

the WUE Bit

The timing of WUE and RCIF events may cause some

confusion when it comes to determining the validity of

received data. As noted, setting the WUE bit places the

USART in an Idle mode. The wake-up event causes a

receive interrupt by setting the RCIF bit. The WUE bit

is cleared after this when a rising edge is seen on

RX/DT. The interrupt condition is then cleared by read-

ing the RCREG register. Ordinarily, the data in RCREG

will be dummy data and should be discarded.

The fact that the WUE bit has been cleared (or is still

set) and the RCIF flag is set should not be used as an

indicator of the integrity of the data in RCREG. Users

should consider implementing a parallel method in

firmware to verify received data integrity.

To assure that no actual data is lost, check the RCIDL

bit to verify that a receive operation is not in process. If

a receive operation is not occurring, the WUE bit may

then be set just prior to entering the Sleep mode.

FIGURE 18-7:

AUTO-WAKE-UP BIT (WUE) TIMINGS DURING NORMAL OPERATION

FIGURE 18-8:

AUTO-WAKE-UP BIT (WUE) TIMINGS DURING SLEEP

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

OSC1

WUE bit

RX/DT Line

RCIF

Cleared due to User Read of RCREG

Note:

The USART remains in Idle while the WUE bit is set.

Auto-Cleared

Bit Set by User

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

OSC1

WUE bit

RX/DT Line

RCIF

Cleared due to User Read of RCREG

Sleep Command Executed

Note 1:

If the wake-up event requires long oscillator warm-up time, the auto-clear of the WUE bit can occur while the stposc signal is still active.

This sequence should not depend on the presence of Q clocks.

The USART remains in Idle while the WUE bit is set.

Sleep Ends

Auto-Cleared

Note 1

Bit Set by User

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