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DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Servicing the Host Interface
16
(ISR), and RXH/RXL may be accessed at any time by the Host Processor (using
HA0-HA2, HRW/HRD, HDS/HWR, and HCS) 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 HI8 DMA mode by clearing the HM1 and HM0 bits and
clearing TREQ in the Interface Control Register (ICR).
16.10.9.3 DSP-to-Host Interface Action
The following procedure outlines the steps that the HI8 hardware takes to transfer DMA
data from DSP memory to the Host data bus.
1. The transmit exception is triggered when HTIE = 1 and HTDE = 1, for Interrupt
mode transfers, or when TDMAE = 1 and HTDE = 1, for on-chip DMA
transfers. The exception routine software, or on-chip DMA, writes the data word
into HTX.
2. Transfer the HTX register to the receive byte registers RXH/RXL when they are
empty (RXDF = 0). This automatically occurs. Load the Host DMA address
counter from HM1 and HM0. This action sets HTDE = 1 and trigger another DSP
transmit exception to write HTX.
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
asserted. Deassert the Host Request HREQ pin.
5. If the highest register address has not been reached (i.e., RXDF = 1),
post-increment the HI8 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 is executed.
Note:
The HOST –> DSP data transfers can occur normally in the channel not used
for DMA except when the Host must use polling and not interrupts.
Note:
The transfer of data from the HTX register to the RXH/RXL registers
automatically loads the HI8 DMA address counter from the HM1 and HM0 bits
when in DMA DSP-to-Host mode.