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DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Servicing the Host Interface
16
The Host Processor interrupts are external and use the HREQ pin. HREQ is normally
connected to the Host Processor maskable interrupt input. The Host Processor
acknowledges host interrupts by executing an interrupt service routine. The two LSBs
(RXDF and TXDE) of the Interrupt Status Register may be tested by the Host Processor to
determine the interrupt source. Please refer to Figure 16-12. The Host Processor interrupt
service routine must read or write the appropriate HI8 register in order to clear the
interrupt. HREQ is deasserted when one of the following occurs:
The enabled request is cleared or masked
The DSP is reset
In the case where the Host Processor is a member of the MC680XX family, servicing the
interrupt starts by asserting HREQ to interrupt the processor. The Host Processor then
acknowledges the interrupt by asserting HACK. When HREQ and HACK are
simultaneously asserted, the contents of the IVR are placed on the Host data bus. This
vector tells the Host Processor which routine to use to service the HREQ interrupt.
16.10.9 Host Side DMA Mode Operation
The Host DMA mode allows the transfer of 8-bit or 16-bit data between the DSP HI8 and
an external DMA controller. The HI8 provides the synchronization logic between the two
asynchronous processor systems. The DSP Side of the interface is serviced by any
appropriate servicing mechanism such as polling, interrupts, or DMA transfer.
The external DMA controller provides the transfers between the DSP HI8 registers and the
external DMA memory. The external DMA controller must provide the address to the
external DMA memory. The address of the selected HI8 register is provided by a DMA
address counter in the DSP HI8.
16.10.9.1 Host-to-DSP – Host Interface Action
The following four procedure outlines the steps the HI8 hardware takes to transfer DMA
data from the Host data bus to DSP memory.
1. Assert the Host Request HREQ output pin when the transmit byte registers
TXH/TXL are empty. This always occurs in Host-to-DSP DMA mode when
TXDE = 1.
2. Write the selected transmit byte register from the Host data bus when the HACK
input pin is asserted by the DMA controller. Deassert the HREQ pin.