Motorola DSP56009 User Manual page 178

Serial Host Interface
SHI Programming Considerations
In a receive session, only the receive path is enabled and the HTX-to-IOSR transfers
are inhibited. If the HRNE status bit is set, the HRX FIFO contains valid data, which
may be read by the DSP. When the HRX FIFO is full, the SHI suspends the serial
clock just before acknowledge. In this case, the clock will be reactivated when the
FIFO is read (the SHI gives an ACK = 0 and proceeds receiving) or when HIDLE is set
(the SHI gives ACK = 1, generates the Stop event, and ends the receive session).
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5.6.4.2 Transmit Data In I C Master Mode
A transmit session is initiated if the R/W direction bit of the transmitted slave device
address byte is cleared. Following a transmit initiation, the IOSR is loaded from HTX
(assuming HTX is not empty) and its contents are shifted out, MSB-first, on the SDA
line. Following each transmitted byte, the SHI controller samples the SDA line at the
ninth clock pulse, and inspects the ACK status. If the transmitted byte was
acknowledged (ACK = 0), the SHI controller continues transmitting the next byte.
However, if it was not acknowledged (ACK = 1), the HBER status bit is set to inform
the DSP side that a bus error (or overrun, or any other exception in the slave device)
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has occurred. Consequently, the I C master device generates a Stop event and
terminates the session.
HTX contents are transferred to the IOSR when the complete word (according to
HM0–HM1) has been shifted out. It is, therefore, the responsibility of the
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programmer to select the right number of bytes in an I C frame so that they fit in a
complete number of words. Remember that for this purpose, the slave device address
byte does not count as part of the data.
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 the 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, the SHI will suspend the serial clock until new data is written
into HTX (when the SHI proceeds with the transmit session) or HIDLE is set (the SHI
reactivates the clock to generate the Stop event and terminate the transmit session).
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