Motorola DSP56012 - Manual (Page 175)

Serial Host Interface
SHI Programming Considerations
MOTOROLA DSP56012 User’s Manual 5-27
5.7.3.2 Transmit Data In I
2
C Slave Mode
A transmit session is initiated when the personal slave device address has been
correctly identified and the R/W bit of the received slave device address byte has
been set. Following a transmit initiation, the IOSR is loaded from HTX (assuming the
latter was 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 and transmits the next byte. However, if it
was not acknowledged (ACK = 1), the transmit session is stopped and the SDA line is
released. Consequently, the external master device may generate a stop event in
order to terminate the session.
HTX contents are transferred to IOSR when the complete word (according to
HM0–HM1) has been shifted out. It is, therefore, the responsibility of the
programmer to select the correct number of bytes in an I
2
C frame so that they fit in a
complete number of words. For this purpose, the slave device address byte does not
count as part of the data, and therefore, it is treated separately.
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
2
C master device. Connecting the HREQ line between two
SHI-equipped DSPs, one operating as an I
2
C master device and the other as an I
2
C
slave device, enables full hardware handshaking.
5.7.4 I
2
C Master Mode
The I
2
C Master mode is entered by enabling the SHI (HEN = 1), selecting the I
2
C
mode (HI
2
C = 1) and selecting the Master mode of operation (HMST = 1). Before
enabling the SHI as an I
2
C master, the programmer should program the appropriate
clock rate in HCKR.
When configured in the I
2
C Master mode, the SHI external pins operate as follows: