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DAN-134 Datasheet(PDF) 1 Page - Exar Corporation |
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DAN-134 Datasheet(HTML) 1 Page - Exar Corporation |
1 / 6 page 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 HARDWARE DIFFERENCES • 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 BUS TIMING DIFFERENCES • 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. FIGURE 1. CRYSTAL OSCILLATOR CIRCUITRY DIFFERENCES 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 |
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