EXAR Corporation 48720 Kato Road, Fremont CA, 94538 • (510) 668-7000 • FAX (510) 668-7017 • www.exar.com • uarttechsupport@exar.com

DATA COMMUNICATIONS APPLICATION NOTE

DAN134

June 2002

EXAR’S ST16C654 AND XR16C854 COMPARED WITH TI’S TL16C754B

Author: PY

1.0

INTRODUCTION

This application note describes the major difference between Exar’s ST16C654 and XR16C854 with TI’s

TL16C754B. These devices are similar, with a few hardware, bus timing and firmware-related differences.

1.1

• The TI TL16C754B and Exar’s ST16C654 and XR16C854 are all available in the 68-pin PLCC package.

The Exar QUARTs are also available in the 64-pin TQFP and 100-pin QFP packages while the TI QUART is

available in a 80-pin QFP package. The Exar and TI QUART are pin-to-pin compatible in the 68-pin PLCC

package.

• The oscillator circuitry is similar, but there are some differences when using a crystal oscillator and when

using an external clock. See Figure 1below for the differences in the oscillator circuitry for a crystal oscillator.

When using an external clock input for frequencies greater than 24 MHz, the Exar QUARTs will require a 2K

pull-up resistor on the XTAL2 pin.

1.2

• The TL16C754B requires that the -CS pin is asserted first before the -IOR or -IOW pin and the -IOR or -IOW

pin must be de-asserted before the -CS pin is de-asserted. During a read, the Exar DUARTs can have either

the -CS or the -IOR signal asserted first and have either signal be de-asserted first. The signals are wire-

ORed in the Exar DUARTs, therefore the second signal asserted will initiate the read cycle and the first signal

de-asserted terminates the read cycle. The same is true during a write for -CS and -IOW. The flexibility of

the Exar DUARTs timing can be important in DSP, ARM, and MIPS designs.

C1

10-30 pF

C2

40-60 pF

Y1

1.8432

MHz

to

16

MHz

R1

1.5 K

Ω

R2

1 ΜΩ

XTAL1

XTAL2

C1

22-47 pF

C2

22-47 pF

Y1

1.8432

MHz

to

24

MHz

R1

0-120

Ω

(Optional)

R2

500 ΚΩ − 1 ΜΩ

XTAL1

XTAL2

Exar QUART

TL16C754B