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A.1 Design Considerations

Section A.1.1, “Boot Source Selection,” through Section A.1.3, “No Load Option,” describe hardware and software considerations involving choice of boot source and bootstrap operation when the COP (Computer Operating Properly) is enabled. In addition, a special No Load option is described.

A.1.1 Boot Source Selection

The DSP56824 bootstrap program provides a simple and flexible manner of selecting the boot source, which can be either SPI0 or Port A.

A.1.1.1 Bootstrapping from SPI0

To select SPI0 as the boot source, bit 15 (D[15]) of P:$C000 should be sampled as zero during the bootstrap program execution. If no external memory is mapped at location P:$C000, a pull-down resistor connected to pin D15 provides the correct signal.

In some cases, data can be loaded through SPI0 to external SRAM mapped to the top 32K of external program space, including location P:$C000. In this case, a pull-down resistor will not work since it will be overridden by the value read from location P:$C000 in SRAM. To solve this problem, the bootstrap program writes zero to this location before reading it, so no additional external logic is required.

A.1.1.2 Bootstrapping from Port A

To select Port A as the boot source, bit 15 (D[15]) of P:$C000 should be sampled as one during the bootstrap program execution. If no external memory is mapped to the upper byte at location P:$C000, a pull-up resistor connected to pin D15 provides the correct signal. This should be done if byte-wide EPROM is used as the source from which data is downloaded.

A.1.2 COP Reset

Special attention must be taken if the COP timer is to be used in an application where the DSP56824 comes out of reset in Mode 1. The COP timer is typically used as a watchdog-like timer to guard processor activity, providing an automatic reset signal if a failure occurs. Once started, the COP timer must be reset by software on a regular basis so that it never reaches its time-out value.

If the COP timer reaches its time-out value, an internal reset is generated within the chip and the COP reset vector is fetched. On a COP reset, the original operating mode that was latched from the pins on hardware reset specifies the location of the COP reset vector. For example, if the chip initially comes out of hardware reset in Mode 1, and later the mode is switched to Mode 3, the COP reset vector will be the Mode 1 COP reset vector, P:$7F82, which was the original mode when exiting reset.

COP reset in Mode 1 is essentially identical to hardware reset, as the boot ROM code is initiated. In the case when bootstrapping originally occurred via SPI0, the system needs a way to detect that a COP reset has occurred so the program is resent to the DSP via SPI0.

A-2

DSP56824 User’s Manual

 

A.1.3 No Load Option

It is also possible to use the bootstrap program to jump to a particular location in program memory to begin execution without loading any program RAM. This can be accomplished by insuring that the first 2 bytes read, which select the number of program words to be loaded, are both zero.

 

Bootstrap Program

A-3

Соседние файлы в папке DSP568xx