Freescale-semiconductor DSP56366 User Manual download pdf (Page 147)

SHI Programming Considerations

DSP56366 24-Bit Digital Signal Processor User Manual, Rev. 4

Freescale Semiconductor 7-23

may be used to interrupt the external I

C master device. Connecting the HREQ line between two

SHI-equipped DSPs, one operating as an I

C master device and the other as an I

C slave device, enables

full hardware handshaking.

7.7.4 I

C Master Mode

The I

C master mode is entered by enabling the SHI (HEN=1), selecting the I

C mode (HI

C=1) and

selecting the master mode of operation (HMST=1). Before enabling the SHI as an I

C master, the

programmer should program the appropriate clock rate in HCKR.

When configured in the I

C 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 I

C master mode, a data transfer session is always initiated by the DSP by writing to the HTX

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 I

C 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 I

C 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

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