DE1-SOC COMPUTER SYSTEM WITH NIOS II For Quartus II 15.0
3.1 Interrupts from Parallel Ports
Parallel ports implemented in the FPGA in the DE1-SoC Computer were illustrated in Figure 2, which is reproduced
as Figure 15. As the figure shows, parallel ports that support interrupts include two related registers at the addresses
Base + 8 and Base + C. The Interruptmask register, which has the address Base + 8, specifies whether or not an
interrupt signal should be sent to the processor when the data present at an input port changes value. Setting a bit
location in this register to 1 allows interrupts to be generated, while setting the bit to 0 prevents interrupts. Finally,
the parallel port may contain an Edgecapture register at address Base + C. Each bit in this register has the value 1 if
the corresponding bit location in the parallel port has changed its value from 0 to 1 since it was last read. Performing
a write operation to the Edgecapture register sets all bits in the register to 0, and clears any associated interrupts.
Address
0214331 30
. . .
Base
Base + 8
Base + C
Base + 4
Input or output data bits
Direction bits
Data register
Direction register
Interruptmask register
Edgecapture register
Edge bits
Mask bits
Direction bits
Figure 15. Registers used for interrupts from the parallel ports.
3.1.1 Interrupts from the Pushbutton Switches
Figure 6 showed the registers associated with the pushbutton parallel port. The Interruptmask register allows inter-
rupts to be generated when a key is pressed. Each bit in the Edgecapture register is set to 1 by the parallel port when
the corresponding key is pressed. An interrupt service routine can read this register to determine which key has been
pressed. Writing any value to the Edgecapture register deasserts the interrupt signal being sent to the processor and
sets all bits of the Edgecapture register to zero.
3.2 Interrupts from the JTAG UART
Figure 10 showed the data and Control registers of the JTAG UART. As we said in Section 2.5, RAVAIL in the Data
register gives the number of characters that are stored in the receive FIFO, and WSPACE gives the amount of unused
space that is available in the transmit FIFO. The RE and WE bits in Figure 10 are used to enable processor interrupts
associated with the receive and transmit FIFOs. When enabled, interrupts are generated when RAVAIL for the receive
FIFO, or WSPACE for the transmit FIFO, exceeds 7. Pending interrupts are indicated in the Control register’s RI and
WI bits, and can be cleared by writing or reading data to/from the JTAG UART.
3.3 Interrupts from the FPGA Interval Timer
Figure 14, in Section 2.6, shows six registers that are associated with the interval timer. As we said in Section 2.6,
the TO bit in the Status register is set to 1 when the timer reaches a count value of 0. It is possible to generate an
interrupt when this occurs, by using the ITO bit in the Control register. Setting the ITO bit to 1 causes an interrupt
Altera Corporation - University Program
2015
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