23. Universal Sychronous Asynchronous Receiver and Transmitter (USART)

The Panel-Card has up to three independent USARTs, not including the debug unit.

The Universal Synchronous Asynchronous Receiver Transceiver (USART) provides one full duplex universal synchronous asynchronous serial link. Data frame format is widely programmable (data length, parity, number of stop bits) to support a maximum of standards. The receiver implements parity error, framing error and overrun error detection. The receiver time-out enables handling variable-length frames and the transmitter timeguard facilitates communications with slow remote devices. Multidrop communications are also supported through address bit handling in reception and transmission.

The USART supports the connection to the Peripheral DMA Controller, which enables data transfers to the transmitter and from the receiver. The PDC provides chained buffer management without any intervention of the processor.

Six different modes are implemented within the USARTs:

  • Normal (standard RS232 mode)

  • RS485

  • Hardware Handshaking

  • ISO7816 Protocol: T=0 or T=1

  • IrDA

RS485.  In RS485 operating mode the RTS pin is automatically driven high during transmit operations. If RTS is connected to the "enable" line of the RS485 driver, the driver will thus be enabled only during transmit operations.

Hardware Handshaking.  The hardware handshaking feature enables an out-of-band flow control by automatic management of the pins RTS and CTS. The receive DMA channel must be active for this mode. The RTS signal is driven high if the receiver is disabled or if the DMA indicates a buffer full condition. As the RTS signal is connected to the CTS line of the connected device, its transmitter is thus prevented from sending any more characters.

ISO7816.  The USARTs have an ISO7816-compatible mode which permits interfacing with smart cards and Security Access Modules (SAM). Both T=0 and T=1 protocols of the ISO7816 specification are supported.

IrDA.  The USART features an infrared (IrDA) mode supplying half-duplex point-to-point wireless communication. It includes the modulator and demodulator which allows a glueless connection to the infrared transceivers. The modulator and demodulator are compliant with the IrDA specification version 1.1 and support data transfer speeds ranging from 2.4 kb/s to 115.2 kb/s.

Signals of the Serial Interfaces.  All UARTs/USARTs have one receiver and one transmitter data line (full duplex). Not all USARTs are implemented with full modem control lines. Furthermore the available lines depend largely on the used multiplexing. Most modem control lines can be implemented with standard digital ports.

Hardware Interrupts.  There are several interrupt sources for each USART:

  • Receive: RX Ready, (DMA) Buffer Full, End of Receive Buffer

  • Transmit: TX Ready, (DMA) Buffer Empty, End of Transmit Buffer, Shift Register Empty

  • Errors: overrun, parity, framing, and timeout errors

  • Handshake: the status of CTS has changed

  • Break: the receiver has detected a break condition on RXD

  • NACK: non acknowledge (ISO7816 mode only)

  • Iteration: the maximum number of repetitions has been reached (ISO7816 mode only)

Please refer to the chapter about the DMA unit (PDC) for a description of the "Buffer Full" and "End of Receive / Transmit Buffer" events.