SPI/MICROWIRE-Compatible UART with Integrated True Fail Safe RS-485/RS-422 Transceivers

The MAX3140 is a complete universal asynchronous receiver-transmitter (UART) and a true fail-safe RS-485/RS-422 transceiver combined in a single 28-pin QSOP package for space-, cost-, and power-constrained applications. The MAX3140 saves additional board space as well as microcontroller (µC) I/O pins by featuring an SPI™/QSPI™/MICROWIRE™-compatible serial interface. It is pin-programmable for configuration in all RS-485/RS-422 networks.

The MAX3140 includes a single RS-485/RS-422 driver and receiver featuring true fail-safe circuitry, which guarantees a logic-high receiver output when the receiver inputs are open or shorted. This feature provides immunity to faults without requiring complex termination. The MAX3140 provides software-selectable control of half- or full-duplex operation, data rate, slew rate, and transmitter and receiver phase. The RS-485 driver slew rate is programmable to minimize EMI and results in maximum data rates of 115kbps, 500kbps, and 10Mbps. Independent transmitter/receiver phase control enables software correction of twisted-pair polarity reversal. A 1/8-unit-load receiver input impedance allows up to 256 transceivers on the bus.

The MAX3140's UART includes an oscillator circuit derived from an external crystal, and a baud-rate generator with software-programmable divider ratios for all common baud rates from 300 baud to 230k baud. The UART features an 8-word-deep receive FIFO that minimizes processor overhead and provides a flexible interrupt with four maskable sources, including address recognition on 9-bit networks. Two control lines are included for hardware handshaking-one input and one output.

The MAX3140 operates from a single +5V supply and typically consumes only 645µA with the receiver active. Hardware-invoked shutdown reduces supply current to only 20µA. The UART and RS-485/RS-422 functions can be used together or independently since the two functions share only supply and ground connections (the MAX3140 is hardware- and software-compatible with the MAX3100 and MAX3089).

Applications

* Embedded Systems * HVAC and Building Control * Industrial Control Local Area Networks * Intelligent Instruments * Point-of-Sale (POS) Devices * Transceivers for EMI-Sensitive Applications

* Integrated UART and RS-485/RS-422 Transceiver in a Single 28-Pin QSOP

* SPI/MICROWIRE-Compatible Interface Saves µC I/O Pins

* True Fail-Safe Receiver Output Eliminates Complex Network Termination

* Pin-Programmable RS-485/RS-422 Features

* Half/Full-Duplex Operation

* Slew-Rate Limiting for Reduced EMI

* 115kbps/500kbps/10Mbps Data Rates

* Receiver/Transmitter Phase for Twisted-Pair Polarity Reversal

* Full-Featured UART

* Programmable Up to 230k baud with a 3.6864MHz Crystal

* 8-Word Receive FIFO Minimizes Processor Overhead

* 9-Bit Address-Recognition Interrupt

* Allows Up to 256 Transceivers on the Bus

* Low 20µA Hardware Shutdown Mode

* Hardware/Software-Compatible with MAX3100 and MAX3089