The PC16552D is a dual version of the PC16550D Universal Asynchronous Receiver/Transmitter (UART). The two serial channels are completely independent except for a common CPU interface and crystal input. On power-up both channels are functionally identical to the 16450*. Each channel can operate with on-chip transmitter and receiver FIFOs (FIFO mode) to relieve the CPU of excessive software overhead. In FIFO mode each channel is capable of buffering 16 bytes (plus 3 bits of error data per byte in the RCVR FIFO) of data in both the transmitter and receiver. All the FIFO control logic is on-chip to minimize system overhead and maximize system efficiency.

Signalling for DMA transfers is done through two pins per channel (TXRDY# and RXRDY#). The RXRDY# function is multiplexed on one pin with the OUT 2# and BAUDOUT functions. The CPU can select these functions through a new register (Alternate Function Register).

Each channel performs serial-to-parallel conversion on data characters received from a peripheral device or a MODEM, and parallel-to-serial conversion on data characters received from the CPU. The CPU can read the complete status of each channel at any time. Status information reported includes the type and condition of the transfer operations being performed by the DUART, as well as any error conditions (parity, overrun, framing, or break interrupt).

The DUART includes one programmable baud rate generator for each channel. Each is capable of dividing the clock input by divisors of 1 to (216 - 1), and producing a 16 × clock for driving the internal transmitter logic. Provisions are also included to use this 16 × clock to drive the receiver logic. The DUART has complete MODEM-control capability, and a processor-interrupt system. Interrupts can be programmed to the user's requirements, minimizing the computing required to handle the communications link.

The DUART is fabricated using National Semiconductor's advanced M2CMOS™.

• Dual independent UARTs
• Capable of running all existing 16450 and PC16550D software
• After reset, all registers are identical to the 16450 register set
• Read and write cycle times of 84 ns
• In the FIFO mode transmitter and receiver are each buffered with 16-byte FIFOs to reduce the number of interrupts presented to the CPU
• Holding and shift registers in the 16450 Mode eliminate the need for precise synchronization between the CPU and serial data
• Adds or deletes standard asynchronous communication bits (start, stop, and parity) to or from the serial data
• Independently controlled transmit, receive, line status, and data set interrupts
• Programmable baud generators divide any input clock by 1 to (216 - 1) and generate the 16 × clock
• MODEM control functions (CTS, RTS, DSR, DTR, RI, and DCD)
• Fully programmable serial-interface characteristics:
  • 5-, 6-, 7-, or 8-bit characters
  • Even, odd, or no-parity bit generation and detection
  • 1-, 1½-, or 2-stop bit generation
  • Baud generation (DC to 1.5M baud) with 16 × clock
• False start bit detection
• Complete status reporting capabilities
• TRI-STATE® TTL drive for the data and control buses
• Line break generation and detection
• Internal diagnostic capabilities:
  • Loopback controls for communications link fault isolation
  • Break, parity, overrun, framing error simulation
• Full prioritized interrupt system controls

*Can also be reset to 16450 Mode under software control.

†Note: This part is patented.

TRI-STATE® is a registered trademark of National Semiconductor Corporation

M2CMOS™ is a trademark of National Semiconductor Corporation