Embedded Pentium® Processor-Based POS Terminal Sample Design

Table 7. SYSOPT Setting Options

6.4.6Additional Serial Ports with SMSC 37C669 Super I/O

An additional Super I/O, SMSC 37C669 is used on the board to add two extra serial ports COM3 and COM4. The parallel, IDE, and floppy interfaces are not used on this design. This configuration was chosen over separate 16550’s because of cost and board space considerations.

Note: Each power pin must be individually decoupled with 0.1 µF capacitors.

All inputs to unused devices (i.e., parallel, IDE and floppy) are pulled to their inactive state. The value of the pull-up/pull-down resistor is chosen to keep the inputs within 0.5 V of the supply rails in order to minimize current consumption. Each pin has 10 µA of current leakage.

The FDC37C669 provides 11 decoded address lines, AEN, plus one chip-select input. Since the FDC37C669 can only decode the lower 11 address bits and AEN, an OR-gate is used to decode addresses A11–A15. The output of the OR-gate is low when A11–A15 are low, therefore asserting the FDC37C669 chip select. This chip select, AEN, and 11-bit address decode provides the full 16bit ISA decode necessary.

6.4.6.1Design Note

At the trailing edge of a hardware reset, the SYSOPT input is latched to determine the configuration base address. Refer to Table 8 for SYSOPT settings.

Table 8. SYSOPT Settings

Note: If you are using TTL RS232 drivers, use a 1 KΩ pull-down resistor. If using CMOS RS232 drivers, use a 10 KΩ pull-down resistor. The board uses TTL drivers, therefore SYSOPT should be pulled