4.2 Enabling the Special Registers
The QSC(LP)-100 contains two unique registers, an Interrupt Status Register and an Options Register. These registers are enabled when the SPAD jumper (J6) is removed (factory default). They replace the UART Scratchpad Register on accesses to register address 7.
The Interrupt Status Register and Options Register are accessed through the scratchpad location of any UART. The DLAB bit of the UART (Line Control Register, bit 7) is used to select between the two registers. The most recent write of a DLAB bit in any UART selects between the two registers as shown in Figure 7.
| DLAB Bit | SPAD Jumper | Register selected for |
| address 7 accesses |
| | |
| 0 | removed | Interrupt Status Register |
| 1 | removed | Options Register |
| X | applied | Scratchpad Registers |
Figure 7 --- DLAB bit selects between special registers
4.3 Interrupt Status Register
The read-only Interrupt Status Register can be used to quickly identify which serial ports require servicing after an interrupt. Reading the Interrupt Status Register will return the interrupt status of the entire QSC(LP)-100, as shown in Figure 8. The individual bits are cleared as the interrupting ports are serviced. The interrupt service routine should ensure that the interrupt status register reads zero before exiting.
Bit | Description |
| |
7 (MSB) | 0 (not used) |
6 | 0 (not used) |
| |
5 | 0 (not used) |
4 | 0 (not used) |
| |
3 | Port 4 --- 1 if interrupt pending |
| |
2 | Port 3 --- 1 if interrupt pending |
1 | Port 2 --- 1 if interrupt pending |
| |
0 | Port 1 --- 1 if interrupt pending |
Figure 8 --- Interrupt Status Register
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