Philips Semiconductors

Product specification

SCN68681

Dual asynchronous receiver/transmitter (DUART)

1998 Sep 04

21

tCLK

tCTC

tRx

tTx

X1/CLK

CTCLK

RxC

TxC

+5V

1K

Ω

X1

X2

X2

3.6864MHz

X1

C1

C2

DRIVING FROM

EXTERNAL SOURCE

SCN2681

74LS04

CLOCK

TO OTHER

CHIPS

1K

+5V

CRYSTAL SERIES RESISTANCE3 SHOULD BE LESS THAN 180

Ω

R1: 100K - 1Meg (See design note)

C1 = C2: 0-5pF + (STRAY < 5pF)

tCLK

tCTC

tRx

tTx

OPEN

When using an external clock it is preferred to drive X2 and leave X1 open.

X2 is the input to the internal driver, while X1 is the output.

TO THE REST

OF THE DUART

CIRCUITS

R1

U1

R1 is only required if U1 will not drive to X2 high level.

Previous specifications indicated X2 should be grounded and X1

should be driven. This is still acceptable. It is electrically easier to drive

the amplifier input than to overdrive its output.

R2

R2 = 50k

Ω to 150kΩ

SD00115

Figure 9. Clock Timing

NOTES:

1. INTRN or OP3 – OP7 when used as interrupt outputs.

pronounced and can greatly affect the resultant measurement.

VM

VOL

CSN

(READ OR

WRITE)

OUTPUT

tIR

SD00116

Figure 10. Interrupt Timing

tTXD

tTCS

1 BIT TIME

(1 OR 16 CLOCKS)

TxD

TxC

(INPUT)

TxC

(1X OUTPUT)

SD00117

Figure 11. Transmit Timing