
Servicing the Host Interface
MOTOROLA
Host Interface Eight (HI8)
16-23
Preliminary
16
16.9.2.1 Reserved—Bits 15-8
These bits are reserved or not implemented. They are read as, and written with 0s.
16.9.2.2 Host Command (HC)—Bit 7
This bit is used by the Host Processor to handshake the execution of Host Command
interrupts. Normally, the Host Processor sets HC = 1 to request the Host Command
interrupt from the DSP core. When the Host Command interrupt is acknowledged by the
DSP core the HC bit is cleared by the HI8 hardware. The Host Processor can read the state
of the HC bit to determine when the Host Command has been accepted. After writing HC
= 1 to the CVR, the Host must not write to the CVR again until the HC bit is cleared by the
HI8 hardware. Setting the HC bit causes Host Command pending to be set in the Host
Status Register.
16.9.2.3 CVR Host Vector (HV)—Bits 6–0
These seven bits select the Host Command interrupt offset address in the vector table to be
used by the Host Command interrupt logic. When the Host Command interrupt is
recognized by the DSP interrupt control logic the offset address of the interrupt vector
table taken is 2
× HVI0, 6. The Host can write HC and HV in the same write cycle.
The Host Processor can select any of the 128 possible offset addresses of the interrupt
vector table in the DSP by writing this address divided by 2, and equal to the vector
number, into HV bits. This means the Host Processor can force any of the existing
interrupt handlers, SW interrupts such as ESSI, SCI, IRQA, or IRQB and can use any of
the reserved or otherwise unused interrupt vectors.
16.10 Servicing the Host Interface
The HI8 can be serviced by using one of the following protocols:
Polling
Interrupts
Host DMA
From the Host Processor viewpoint, the service consists of making a data transfer because
this is the only way to reset the appropriate status bits.
16.10.1 Interface Status Register (ISR)
The Interface Status Register (ISR) is an 8-bit read-only status register used by the Host
Processor to interrogate the status and flags of the HI8. The Host Processor can write this