SHI Programming Considerations
DSP56366 24-Bit Digital Signal Processor User Manual, Rev. 4
Freescale Semiconductor 7-23
may be used to interrupt the external I2C master device. Connecting the HREQ line between two
SHI-equipped DSPs, one operating as an I2C master device and the other as an I2C slave device, enables
full hardware handshaking.
7.7.4 I2C Master Mode
The I2C master mode is entered by enabling the SHI (HEN=1), selecting the I2C mode (HI2C=1) and
selecting the master mode of operation (HMST=1). Before enabling the SHI as an I2C master, the
programmer should program the appropriate clock rate in HCKR.
When configured in the I2C master mode, the SHI external pins operate as follows:
• SCK/SCL is the SCL open drain 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.
In the I2C 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 is 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) is shifted out, independent of the word length defined by
the HM[1:0] bits.
In order for the DSP to initiate a data transfer the following actions are to be performed:
• The DSP tests the HIDLE status bit.
• If the HIDLE status bit is set, the DSP writes the slave device address and the R/W bit to the most
significant byte of HTX.
• The SHI generates a start event.
• The SHI transmits one byte only, internally samples the R/W direction bit (last bit), and
accordingly initiates a receive or transmit session.
• The SHI inspects the SDA level at the ninth clock pulse to determine the ACK value. If
acknowledged (ACK = 0), it starts its receive or transmit session according to the sampled R/W
value. If not acknowledged (ACK = 1), the HBER status bit in HCSR is set, which causes an SHI
Bus Error interrupt request if HBIE is set, and a stop event is generated.
The HREQ input pin is ignored by the I2C master device if HRQE[1:0] are cleared, and considered if either
of them is set. When asserted, HREQ indicates that the external slave device is ready for the next data
transfer. As a result, the I2C master device sends clock pulses for the full data word transfer. HREQ is
deasserted by the external slave device at the first clock pulse of the next data transfer. When deasserted,
HREQ prevents the clock generation of the next data word transfer until it is asserted again. Connecting