5.14.3 Dsp To Host Interface Action - Motorola DSP56156 Manual

3. The DSP's destination pointer used in the DMA exception handler (an address register
for example) must be initialized and HRIE must be set to enable the HRDF interrupt.
This could be done with a separate Host Command exception routine in the DSP.
HREQ output pin will be asserted immediately by the DSP hardware which begins the
DMA transfer.
4. Perform other tasks until interrupted by the DMA controller DMA Complete interrupt.
The DSP Interrupt Control Register (ICR), the Interrupt Status Register (ISR), and
RXH:RXL may be accessed at any time by the host processor (using HA0-HA2, HR/
W, and HEN) but the Transmit Byte Registers (TXH:TXL) may not be accessed until
the DMA mode is disabled.
5. Terminate the DMA controller channel to disable DMA transfers.
6. Terminate the DSP HI DMA mode by clearing the HM1 and HM0 bits and clearing
TREQ in the Interrupt Control Register (ICR).

5.14.3 DSP to Host Interface Action

The following procedure outlines the steps that the HI hardware takes to transfer DMA
data from DSP memory to the Host Data Bus.
1. The transmit exception will be triggered when HTIE=1 and HTDE=1. The exception
routine software will write the data word into HTX.
2. Transfer the HTX register to the Receive Byte Registers RXH:RXL when they are
empty (RXDF=0). This will automatically occur. Load the DMA address counter from
HM1 and HM0. This action will set HTDE=1 and trigger another DSP transmit excep-
tion to write HTX (i.e., HTDE=0).
3. Assert the Host Request HREQ pin when the Receive Byte Registers are full.
4. Enable the selected Receive Byte register on the Host Data Bus when HACK is as-
serted. Deassert the Host Request HREQ pin.
5. If the highest register address has not been reached (i.e., RXDF=1), postincrement the
DMA address counter to select the next register. Wait until HACK is deasserted then
go to step 3.
6. If the highest register address has been reached (i.e., RXDF=0), wait until HACK is
deasserted then go to step 2. The DSP transmit exception must have written HTX (i.e.,
HTDE=0) before Step 2 will be executed.
NOTES:
• The HOST→ DSP data transfers can occur normally in the channel not used for
DMA except that the HOST must use polling and not interrupts.
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DMA MODE OPERATION
HOST INTERFACE MOTOROLA