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4.2 Enabling the Special Registers
The QSC-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 6.
Scratchpad RegistersappliedX
Options Registerremoved1
Interrupt Status Registerremoved0
Register selected for
address 7 accesses
SPAD JumperDLAB Bit
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-100, as shown in Figure 7. 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.
Port 1 --- 1 if interrupt pending0
Port 2 --- 1 if interrupt pending1
Port 3 --- 1 if interrupt pending2
Port 4 --- 1 if interrupt pending3
0 (not used)4
0 (not used)5
0 (not used)6
0 (not used)7 (MSB)
DescriptionBit
Figure 8 --- Interrupt Status Register
Quatech QSC-100 User's Manual 10