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
DAN131
June 2002
EXAR’S ST16C650 A AND XR16C850 COMPARED WITH TI’S TL16C750
Author: PY
1.0 INTRODUCTION
This application note describes the major difference between Exar’s ST16C650A and XR16C850 with TI’s
TL16C750. These devices are very similar, with a few hardware, firmware-related and bus timing differences.
1.1 H
ARDWARE DIFFERENCES
• The TI TL16C750 and Exar ST16C650A and XR16C850 are all available in the 44-pin PLCC package. The
ST16C650A and XR16C850 are also available in the 40-pin PDIP and 48-pin TQFP packages. Additionally,
the XR16C850 is available in the 52-pin QFP package. The TL16C750 is also available in the 64-pin SQFP
package. In the 44-pin PLCC package, the Exar and TI UARTs are pin-to-pin compatible if pin 34 of the
ST16C650A and XR16C850 is tied to VCC (pin 34 of the TL16C750 is not used).
• 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 ST16C650A and XR16C850
will require a 2K pull-up resistor on the XTAL2 pin.
1.2 B
US TIMING DIFFERENCES
• The TL16C750 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 UART 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 UART, 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 UARTs timing can be important in DSP, ARM, and MIPS designs.
F
IGURE 1. CRYS TAL OSCILLATOR CIRCUITRY DIFFERENCES
C1
10-30 pF
C2
40-60 pF
Y1
1.8432
MHz
to
16
MHz
R1
1.5 KΩ
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
ST16C650A / XR16C850TL16C750
DATA COMMUNICATIONS APPLICATION NOTE
DAN131
2
1.3 FIRMWARE DIFFERENCES
1.3.1 Firmware Differences Between the ST16C650A and TL16C750
The internal registers in the ST16C650A offers more features than the TL16C750 with some differences:
R = Read-Only, W = Write-Only, R/W = Read/Write
T
ABLE 1: ST16C650A AND TL16C750 REGISTER SET DIFFERENCES
A2:A0 R/W ST16C650A TL16C750
LCR Bit-7 = 0
001 R/W Interrupt Enable Register (IER)
• Bit-7 = Auto CTS# Interrupt Enable
• Bit-6 = Auto RTS# Interrupt Enable
• Bit-5 = Xoff Interrupt Enable
Interrupt Enable Register (IER)
• Bit-7 = Not Used
• Bit-6 = Not Used
• Bit-5 = Low Power Mode
010 W FIFO Control Register (FCR)
• Bit-5 = TX FIFO Trigger Level Select Bit-1
• Bit-4 = TX FIFO Trigger Level Select Bit-0
FIFO Control Register (FCR)
• Bit-5 = 64 Byte FIFO Enable
• Bit-4 = Not Used
010 R Interrupt Status Register (ISR)
• Bit-5 = Auto RTS/CTS Interrupt
• Bit-4 = Xoff or Special Character Interrupt
Interrupt Status Register (ISR)
• Bit-5 = 64 Byte FIFO Enabled
• Bit-4 = Not Used
100 R/W Modem Control Register (MCR)
• Bit-7 = BRG Prescaler
• Bit-6 = Infrared Mode Enable
• Bit-5 = INT Type Select
• Bit-3 = OP2 Control/INT Output Enable in PC
Mode
Modem Control Register (MCR)
• Bit-7 = Not Used
• Bit-6 = Not Used
• Bit-5 = Auto RTS/CTS Flow Control Enable
• Bit-3 = OP2 Control
101 W Extra Feature Register (XFR)
• RS485 Output Inversion, XonAny, LSR Inter-
rupt Immediate, RS485 Enable, IR RX Inver-
sion, IR Half-Duplex/Full-Duplex Mode
N/A
110 W Infrared T ransmit Pulsewid th Control Register
(IRPW)
N/A
LCR Bit-7 = 0, DLL = 0x00, DLM = 0x00
000 R Device Revision (DREV) N/A
001 R Device ID (DVID) N/A
LCR = 0xBF
010 R/W Enhanced Featu r e Registe r (EFR)
• Auto RTS/CTS Enable, Enhanced Functions
Enable, Software Flow Control Select
N/A
100 R/W XON1 N/A
101 R/W XON2 N/A
110 R/W XOFF1 N/A
111 R/W XOFF2 N/A