Motorola MSC8101 ADS User's Guide Page 165

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Managing Data Transfers Via Handshaking Protocols
Host Interface (HDI16) 8-13
A similar situation occurs when the host performs multiple reads from the HDI16 port
Receive Word Registers (RX[0–3]). The DSP side uses an appropriate handshaking
protocol to determine whether the 64-bit Host Transmit Register (HOTX) FIFO is not full.
If HOTX is not full, the DSP writes the data to this register. Data is transferred to the
host-side Receive Word Registers (RX[0–3]) only if they are empty (that is, the host has
previously read them). The host can then use any of the available handshaking protocols to
determine when data is ready to be read.
The MSC8101 HDI16 port offers the following handshaking protocols for data transfers
with the host:
Software polling (DSP and host)
DSP interrupts
Host requests
Direct Memory Access (MSC8101 on-device DMA and host DMA)
The following sections discuss several factors that determine which protocol to use,
including:
The amount of data to be transferred
The timing requirements for the transfer
The availability of resources such as processing bandwidth and DMA channels
Recall that the transfers described here occur between the host and the DSP
asynchronously. Each side transfers data at its own pace. However, using an appropriate
handshaking protocol allows data to be transferred at optimal rates. Furthermore, the DSP
and the host can use different handshaking techniques.
8.3.1 Software Polling
Software polling is the simplest handshaking protocol, but it can consume the most
processing power. In software polling, the host or the DSP core reads status bits to
determine the state of the HDI16 registers. While polling these status bits, the DSP or host
consumes processing clocks.
8.3.1.1 DSP Polling
On the DSP-side, four bits are available for polling. In DSP-to-host transfers (host reads),
the DSP core can determine whether the HOTX is empty, partially empty/full, or full. To
determine whether HOTX is empty, the DSP core polls the Host Transmit Empty bit in the
Host Status Register (HSR[13]:HTFE):
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