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HINT bit=0
HINT
signal
is high
is low
HINT signal
HINT bit=1
CPU writes 1
to HINT bit
Host writes 1
to HINT bit
Interrupt
active
CPU writes 0
to HINT bit
No interrupt/
interrupt
cleared
Host writes 0 or 1
to HINT bit
CPU writes 0 or 1
to HINT bit
Host writes 0
to HINT bit
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Interrupts Between the Host and the CPU
Figure 27. CPU-to-Host Interrupt State Diagram
If the CPU writes 1 to the HINT bit of HPIC, the HPI drives the HINT signal low, indicating an interrupt
condition to the host. Before the CPU can use the HINT bit to generate a subsequent interrupt to host, the
host must acknowledge the current interrupt by writing 1 to the HINT bit. When the host does this, the HPI
clears the HINT bit (HINT = 0), and this drives the HINT signal high. The CPU should read HPIC and
ensure HINT = 0 before generating subsequent interrupts. Writes of 0 have no effect on the HINT bit. A
hardware reset immediately clears the HINT bit and thus clears an active CPU-to-host interrupt.
31
SPRUGK7A–March 2009–Revised July 2010 Host Port Interface (HPI)
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