XR68C681
Dual UART with Two Fully Independent Full Duplex Asynchronous Communications Channels
Data Sheets
OBS
(Obsolete)
Overview
| Information | Dual UART with Two Fully Independent Full Duplex Asynchronous Communications Channels |
|---|---|
| Data Bus Interface | Motorola |
| # of Channels | 2 |
| Max Data Rate 5V (Mbps) | 1 |
| Max Data Rate 3.3V (Mbps) | na |
| Max Data Rate 2.5V (Mbps) | na |
| Max Data Rate 1.8V (Mbps) | na |
| Tx FIFO (Bytes) | 1 |
| Rx FIFO (Bytes) | 3 |
| Auto Flow Control | ✔ |
| Auto RS-485 Half-Duplex Control | |
| Multidrop (9-bit) Mode | ✔ |
| Fractional Baud Rate Generator | |
| Power Down Mode | ✔ |
| Supply Voltage Range VCC (V) | 4.5 to 5.5 |
| Auto RTS/CTS | ✔ |
| Package | PLCC-44 |
| FIFO Level Counters | |
| Selectable/ Programable Trigger Levels | |
| IrDA Sup | |
| 5V Tolerant Inputs | |
| Max UART/GPIO Input Voltage (V) | VCC |
| Max UART/GPIO Output Voltage (V) | VCC |
| Temperature Range (°C) | -40 to 85 |
The MaxLinear Dual Universal Asynchronous Receiver and Transmitter (DUART) is a data communications device that provides two fully independent full duplex asynchronous communications channels in a single package. The DUART is designed for use in microprocessor based systems and may be used in a polled or interrupt driven environment.
The XR-68C681 device offers a single IC solution for the 68000 family of microprocessors.
The DUART is fabricated using advanced two layer metal, with a high performance density EPI/CMOS 1.8 process to provide high performance and low power consumption, and is packaged in a 40 pin DIP or a 44 pin PLCC.
- Two Full Duplex, Independent Channels
- Asynchronous Receiver and Transmitter
- Quadruple-Buffered Receiver, Dual Buffered Transmitter
- Programmable Stop Bits in 1/16 Bit Increments
- Internal Bit Rate Generators with 23 Different Bit Rates
- Independent Bit Rate Selection for Each Transmitter and Receiver
- External Clock Capability
- Maximum Bit Rate: 1X Clock - 1Mb/s, 16X Clock - 125Kb/s
- Normal, AUTOECHO, Local LOOPBACK and Remote LOOPBACK Modes
- Multi-function 16 Bit Counter/Timer
- Interrupt Output with Eight Maskable Interrupt Conditions
- Interrupt Vector Output on Acknowledge
- 8 General Purpose Outputs
- 6 General Purpose Inputs with Change of States Detectors on Inputs
- On-chip Oscillator for Crystal
- Standby Mode to Reduce Operating Power
- Compatible with the Motorola MC68681 and the Signetic SCC68692 Devices
- Advanced CMOS Low Power Technology
- Pb-Free, RoHS Compliant Versions Offered
- Multimedia Systems
- Serial to Parallel/Parallel to Serial Converter
- DTE for Modem Communication Systems
Documentation & Design Tools
All Types
Data Sheets
Application Notes
Schematics & Design Files
Product Briefs & Brochures
Software & Utilities
Design Tools & Simulation
User Guides & Manuals
| Type | Title | Version | Date | File Size |
|---|---|---|---|---|
| Data Sheets | XR68C681 CMOS Dual Channel UART (DUART) | 2.1.0 | September 1999 | 1.4 MB |
| Application Notes | DAN-174, Upgrading from XR68C681 to XR68C92 and XR68C192 | 1.0.0 | January 2004 | 40.9 KB |
| Application Notes | DAN-122, XR68C681 vs. SCC68692 | 1.0.0 | June 2002 | 30.9 KB |
| Application Notes | DAN-101, XR68C681 vs. SCN68681 | 1.0.0 | May 2002 | 20.4 KB |
| Application Notes | TAN-012, Comparison between XR68C681 and SCC68692 Duart Devices | 1.0.0 | October 1997 | 50.1 KB |
| User Guides & Manuals | Evaluation Board User's Manual | 2.1.0 | August 2003 | 13.4 KB |
| Software: Drivers | DOS | 1.0.0 | December 2009 | 1.1 MB |
| Product Brochures | Interface Brochure | R02 | November 2024 | 3.6 MB |
| Schematics & Design Files | ISA Eval Board Schematic | 2.1.0 | August 2007 | 105 KB |
Simulation Models
| Package Type | Vcc | Temp | Mode | Version | File |
|---|---|---|---|---|---|
| PLCC | 5V | Commercial | Motorola | 1 | |
| PLCC | 5V | Commercial | Motorola | 1 | |
| PDIP | 5V | Commercial | Motorola | 1 | |
| PDIP | 5V | Commercial | Motorola | 1 | |
| PLCC | 5V | Industrial | Motorola | 1 | |
| PLCC | 5V | Industrial | Motorola | 1 | |
| PDIP | 5V | Industrial | Motorola | 1 | |
| PDIP | 5V | Industrial | Motorola | 1 | |
| CDIP | 5V | Industrial | Motorola | 1 | |
| CDIP | 5V | Industrial | Motorola | 1 |
Quality & RoHS
| Part Number | RoHS | Exempt | RoHS | Halogen Free | REACH | TSCA | MSL Rating / Peak Reflow | Package |
|---|---|---|---|---|---|---|---|
| XR68C681CJ-F | N | Y | Y | Y | Y | L1 / 260ᵒC | PLCC44 |
| XR68C681J-F | N | Y | Y | Y | Y | L1 / 260ᵒC | PLCC44 |
| XR68C681CJTR-F | N | Y | Y | Y | Y | L1 / 260ᵒC | PLCC44 |
Click on the links above to download the Certificate of Non-Use of Hazardous Substances.
Additional Quality Documentation may be available, please Contact Support.
Parts & Purchasing
| Part Number | Pkg Code | Min Temp | Max Temp | Status | Suggested Replacement | PDN |
|---|---|---|---|---|---|---|
| XR68C681CJ-F | PLCC44 | 0 | 70 | OBS | ||
| XR68C681CJTR-F | PLCC44 | 0 | 70 | OBS | ||
| XR68C681CP | PDIP40 | 0 | 70 | OBS | XR68C681CP-F | |
| XR68C681CP-F | PDIP40 | 0 | 70 | OBS | XR68C681CJ-F | |
| XR68C681J-F | PLCC44 | -40 | 85 | OBS | ||
| XR68C681JTR-F | PLCC44 | -40 | 85 | OBS | XR68C681J-F | |
| XR68C681P | PDIP40 | -40 | 85 | OBS | XR68C681P-F | |
| XR68C681P-F | PDIP40 | -40 | 85 | OBS | XR68C681J-F |
Part Status Legend
Active - the part is released for sale, standard product.
EOL (End of Life) - the part is no longer being manufactured, there may or may not be inventory still in stock.
CF (Contact Factory) - the part is still active but customers should check with the factory for availability. Longer lead-times may apply.
PRE (Pre-introduction) - the part has not been introduced or the part number is an early version available for sample only.
OBS (Obsolete) - the part is no longer being manufactured and may not be ordered.
NRND (Not Recommended for New Designs) - the part is not recommended for new designs.
Active - the part is released for sale, standard product.
EOL (End of Life) - the part is no longer being manufactured, there may or may not be inventory still in stock.
CF (Contact Factory) - the part is still active but customers should check with the factory for availability. Longer lead-times may apply.
PRE (Pre-introduction) - the part has not been introduced or the part number is an early version available for sample only.
OBS (Obsolete) - the part is no longer being manufactured and may not be ordered.
NRND (Not Recommended for New Designs) - the part is not recommended for new designs.
Packaging
| Pkg Code | Details | Quantities | Dimensions |
|---|---|---|---|
| PDIP40 |
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| PLCC44 |
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Notifications
FAQs & Support
Search our list of FAQs for answers to common technical questions.
For material content, environmental, quality and reliability questions review the Quality tab or visit our Quality page.
For ordering information and general customer service visit our Contact Us page.
Submit a Technical Support Question As a New Question
LSR bit-6 is a superset of LSR bit-5. The transmitter consists of a TX FIFO (or THR only when FIFOs are not enabled) and a Transmit Shift Register (TSR). When LSR bit-5 is set, it indicates that the TX FIFO (or THR) is empty, however there may be data in the TSR. When LSR bit-6 is set, it indicates that the transmitter (TX FIFO + TSR) is completely empty.
You can tell by reading LSR bit-5 or bit-6. If they are '0', then the transmit interrupt was generated by the trigger level. If they are '1', then the transmit interrupt was generated by the TX FIFO becoming empty. For enhanced UARTs, you can just read the FIFO level counters.
An RX Data Ready interrupt is generated when the number of bytes in the RX FIFO has reached the RX trigger level. An RX Data Timeout interrupt is generated when the RX input has been idle for 4 character + 12 bits time.
For some UARTs, the RX Data Timeout interrupt has a higher priority and in others, the RX Data Ready interrupt has a higher priority. See the interrupt priority section of the datasheet.
The UART requires a clock and a valid baud rate in order to transmit and receive data. Check that there is a clock signal on the XTAL1 input pin. Also, valid divisors need to be written into the DLL and DLM registers. Most UARTs have random (invalid) values upon power-up.
For most UARTs, the interrupt is generated when the data is ready to be read from the RX FIFO. The are some UARTs that generate the interrupt when the character with the error is received. There are some UARTs that have a register bit to select whether the LSR interrupt is generated immediately or delayed until it is ready to be read.
The UART will enter the sleep mode if the following conditions have been satisfied for all channels:
-Sleep Mode is enabled
-No interrupts are pending
-TX and RX FIFOs are empty
-RX input pin is idling HIGH (LOW in IR mode)
-Valid values in DLL and DLM registers
-Modem input pins are idle (MSR bits 3-0=0x0)
See AN204, UART Sleep Mode for more information on UART Sleep Mode
The UART will wake-up from sleep mode by any of the following conditions on any channel:
-Sleep mode is disabled
-Interrupt is generated
-Data is written into THR
-There is activity on the RX input pin
-There is activity on the modem input pins
If the sleep mode is still enabled and all wake-up conditions have been cleared, it will return to the sleep mode.
See AN204, UART Sleep Mode for more information on UART Sleep Mode
There will be no activity on the XTAL2 output.
See AN204, UART Sleep Mode for more information on UART Sleep Mode
For any UART that has the wake-up indicator interrupt, an interrupt will be generated when the UART wakes up even if no other interrupts are enabled.
See AN204, UART Sleep Mode for more information on UART Sleep Mode
No, Auto RTS and Auto CTS are independent. Auto RTS is toggled by the UART receiver. Auto CTS is monitored by the UART Transmitter.
No, Auto RTS and Auto CTS will work normally without the interrupts enabled.
No, software flow control characters are not loaded into the RX FIFO.
Since 2-character software flow control requires that 2 consecutive flow control characters match before data transmission is stopped or resumes, there is less of a chance that data transmission is stopped because one data byte matched a control character.
Auto RS485 Half-Duplex Control feature overrides the Auto RTS flow control feature if both features use the RTS# output pin. Both features can only be used simultaneously if the Auto RS485 control output is not the RTS# output. For some UARTs, the Auto RS485 control output is not the RTS# output.
Most UARTs use RTS#, however the XR16C850 and XR16C864 use the OP1# output as the Auto RS485 control output. In addition to using the RTS# output as the Auto RS485 control output, the XR16L784, XR16L788 and XR16V798 can use the DTR# output as the Auto RS485 control output.
The polarity of the RS485 control output varies from one UART to another. For some UARTs, an inverter may be required. Some of the newer UARTs have register bits that can change that polarity of the RS485 control output.
In the normal mode, the TX interrupt is generated when the TX FIFO is empty, and there may still be data in the Transmit Shift Register. In the RS485 mode, the TX interrupt is generated when the TX FIFO and the TSR register are both empty.
It is recommended that the FIFO counters at the Scratchpad Register location be used. When transmitting or receiving data, writing to the LCR register could result in transmit and/or receive data errors.
Due to the dynamic nature of the FIFO counters, it is recommended that the FIFO counter registers be read until consecutive reads return the same value.
All of the UARTs that have the IR mode supports up to 115.2Kbps as specified in IrDA 1.0. The newer I2C/SPI UARTs can support up to 1.152Mbps as specified in IrDA 1.1.
For external clock frequencies above 24MHz at the XTAL1 input, a 2K pull-up may be necessary to improve the rise times if there are data transmission errors.
Yes, you can daisy-chain it like that, but only up to 2 times (3 UARTs total in the daisy-chain). The UARTs should be as close as possible.
No, it just has to meet the minimum high and low pulse widths.
Yes, if you are using a UART with a fractional baud rate generator. This provides a divisor feature with a granularity of 1/16, allowing for any baud rate to be generated by any clock frequency, standard or non-standard. Click on the parametric search button of the product family page and find the Fractional Baud Rate Generator column which tells which products have this feature.
They crystal oscillator circuitry is recommended for fundamental frequency crystals only. The maximum frequency for crystals with fundamental frequencies is typically 24MHz. Above that frequency, crystals operate at higher harmonics, which will not work with the recommended crystal oscillator circuitry.
No. It is only required for transmitting and receiving data.
Visit the product page for the part you are interested in. The part's status is listed in the Parts & Purchasing section. You can also view Product Lifecycle and Obsolescence Information including PDNs (Product Discontinuation Notifications).
To visit a product page, type the part into the search window on the top of the MaxLinear website.
In this example, we searched for XRA1201. Visit the product page by clicking the part number or visit the orderable parts list by clicking "Orderable Parts".
The Parts & Purchasing section of the product page shows the Status of all orderable part numbers for that product. Click Show obsolete parts, to see all EOL or OBS products.
It depends on the baud rate. For example, for a start bit, 8 data bits, no stop bit and 1 stop bit, the maximum baud rate deviation is 4.76%. For more information, see https://www.exar.com/appnote/dan108.pdf
Please check that all the following conditions are satisfied first.
- no interrupts pending (ISR bit-0 = 1)
- modem inputs are not toggling (MSR bits 0-3 = 0)
- RX input pin is idling HIGH • divisor (the value in DLL register) is non-zero
- TX and RX FIFOs are empty
Be sure sleep mode bit has been set to 1. If there are multiple UART channels, the sleep conditions must be true for all channels.
See more on Sleep Mode in AN204 UART Sleep Mode.
Yes. Note: some devices do have powersave mode. If UART goes into powersave mode, then the registers are not accessible.
See more on Sleep Mode in AN204 UART Sleep Mode.
Read LSR register to check whether the UART receives the data or not.
- If LSR value is 0x60, it means that either UART receiver FIFO doesn’t receive the data or the data in receiver FIFO has been read out before the read of LSR.
- If LSR value is 0x00, it means data is still in the THR (clock doesn’t oscillate to transmit data).
- If LSR value is 0xFF, it means either UART is in powersave mode or UART is powered off. For those devices with powersave mode, be sure that UARTS are not in powersave mode.
See more on Sleep Mode in AN204 UART Sleep Mode.
- Check whether the register set can be accessed.
- Check whether the crystal is oscillating fully.
- Check whether the data can be transmitted in internal loopback mode.
See more on Sleep Mode in AN204 UART Sleep Mode.
