4.2 Enabling the Special Registers
The DSC-200/300 contains two unique registers, an Interrupt Status Register and
an Options Register. These registers are enabled when the SPAD jumper (J13) 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 8.
Scratchpad Registers
applied
X
Options Register
removed
1
Interrupt Status Register
removed
0
Register selected for
address 7 accesses
SPAD Jumper
DLAB Bit
Figure 8 --- 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 DSC-200/300, as shown in Figure 9. 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 pending
0
Port 2 --- 1 if interrupt pending
1
0 (not used)
2
0 (not used)
3
0 (not used)
4
0 (not used)
5
0 (not used)
6
0 (not used)
7 (MSB)
Description
Bit
Figure 9 --- Interrupt Status Register
Quatech DSC-200/300 User's Manual
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