The 68338 has a Serial Controller Interface (SCI) that provides standard UART functions at rates from 64 to 500 kbaud with advanced error detection circuitry. The SCI supports full or half duplex operation, double buffering, optional parity generation and detection, and wakeup on idle line or address detection.

The CF1 has an onboard MAX3222 dual EIA driver to interface directly to any standard RS-232 terminal or device. Either or both the receive and transmit drivers can be disabled under software control to conserve power or allow connection to alternate RS-485 or RS-422 drivers.

BIOS UART Support: The CF1 BIOS software handles port configuration, polled or buffered receive and transmit, and flow control with several dozen high-level C functions.

One of the example projects that ships on the CF1 Developer's CD demonstrates how to stream incoming RS-232 data to a file on the CompactFlash card at any standard BAUD rate while automatically dropping to less than 2mA whenever the serial line idles. This particular example is 180 lines of C code, over 100 of which are just comments and formatting.

RS-232 Wakeup Call: Both the RXD and CTS signals connect to interrupt request pins on the 68338 to allow wakeup from deep sleep modes from any UART activity. However, in the 10µA SUSPEND mode, power to the CPU is completely off so we add the simple circuit at right to yank on the /WAKE line when a UART character comes in. It may seem odd to tie the emitter of the NPN transistor to VREG, but VREG actually looks like GROUND when the CF1 is in SUSPEND, and this circuit keeps the WAKE line from flailing during normal operation.

2nd UART borrowing spare drivers: All of the CMOS level UART signals are brought out to allow driver replacement, and the RTS and CTS signals can be used for EIA level flow control or reassigned to work as RXD and TXD signals for a second external UART such as a MAX3100 SPI UART shown below. In the next section, you'll see that this same technique can be used to expand to up to 14 additional UARTs.

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