Motorola DSP56009 User Manual page 176

Serial Host Interface
SHI Programming Considerations
In a transmit session, only the transmit path is enabled and the IOSR-to-HRX FIFO
transfers are inhibited. When the HTX transfers its valid data word to IOSR, the
HTDE status bit is set and the DSP may write a new data word to HTX. If both IOSR
and HTX are empty, an underrun condition occurs, setting the HTUE status bit; if
this occurs, the previous word will be retransmitted.
The HREQ output pin, if enabled for transmit (HRQE1–HRQE0 = 10), is asserted
when HTX is transferred to IOSR for transmission. When asserted, HREQ indicates
that the slave device is ready to transmit the next data word. HREQ is deasserted at
the first clock pulse of the next transmitted data word. The HREQ line may be used to
interrupt the external I
SHI-equipped DSPs, one operating as an I
slave device, enables full hardware handshaking.
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5.6.4 I C Master Mode
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The I C Master mode is entered by enabling the SHI (HEN = 1), selecting the I
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mode (HI C = 1) and selecting the master mode of operation (HMST = 1). Before
enabling the SHI as an I
clock rate in HCKR.
When configured in the I
• SCK/SCL is the SCL serial clock output.
• MISO/SDA is the SDA open drain serial data line.
• MOSI/HA0 is the HA0 slave device address input.
• SS/HA2 is the HA2 slave device address input.
• HREQ is the Host Request input.
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In the I C Master mode, a data transfer session is always initiated by the DSP by
writing to the HTX register when HIDLE is set. This condition ensures that the data
byte written to HTX will be interpreted as being a slave address byte. This data byte
must specify the slave device address to be selected and the requested data transfer
direction.
Note: The slave address byte should be located in the high portion of the data word,
whereas the middle and low portions are ignored. Only one byte (the slave
address byte) will be shifted out, independent of the word length defined by
the HM0–HM1 bits.
5-28
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C master device. Connecting the HREQ line between two
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C master, the programmer should program the appropriate
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C Master mode, the SHI external pins operate as follows:
DSP56009 User's Manual
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C master device and the other as an I
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C
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C
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