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4.2 Enabling the Special Registers
The QSCLP-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 QSCLP-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
QSCLP-100 User's Manual 7