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Freescale Semiconductor, Inc.

SSI Reset and Initialization Procedure

ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005

appropriate clock pin. This allows data to be transferred out in periodic intervals in gated clock mode. With an external clock, the SSI waits for a clock signal to be received. Once the clock begins, valid data is shifted in.

NOTE:

In gated clock mode with the clock generated externally, the receive interrupt does not occur until the first serial clock of the following word. This can appear in an application as a one-word shift in a series of received data words.

In general, the bit clock pins must be kept free of timing glitches. If a single glitch occurs, all ensuing transfers will be out of synchronization.

Freescale Semiconductor, Inc.

ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005

8.5 SSI Reset and Initialization Procedure

The SSI is affected by three types of reset:

DSP reset—The DSP reset is generated by asserting either the RESET pin or the Computer Operating Properly (COP) timer reset. The DSP reset clears the SSIEN bit in SCR2, which disables the SSI. All other status and control bits in the SSI are affected as described in Section 8.2, “SSI Programming Model.”

SSI reset—The SSI reset is generated when the SSIEN bit in the SCR2 is cleared. The SSI status bits are preset to the same state produced by the DSP reset. The SSI control bits are unaffected. The control bits in the top half of the SCSR are also unaffected. The SSI reset is useful for selective resetting of the SSI without changing the present SSI control bits and without affecting the other peripherals.

STOP reset—The STOP reset is caused by executing the STOP instruction. While in stop mode, no clock is active in the SSI, which is always powered down in stop mode. The SSI status bits are preset to the same state produced by the DSP reset. The SSI control bits are unaffected. The control bits in the top half of the SCSR are also unaffected.

The correct sequence to initialize the SSI is as follows:

1.Issue a DSP or SSI reset.

2.Program SSI control registers.

3.Set the SSIEN bit in SCR2.

To ensure proper operation of the SSI, the DSP programmer should use the DSP or SSI reset before changing any of the control bits listed in Table 8-7 on page 8-30. These control bits should not be changed during SSI operation.

Synchronous Serial Interface

8-29

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