HOST INTERFACE (HI)
MOTOROLA
PORT B
5 - 67
potentially read the wrong combination. Two solutions would be to 1) read the
bits twice and check for consensus, or 2) hold HEN access for HEN + x clock
cycles so that status bit transitions are stabilized.
4. Overwriting the Host Vector:
The host programmer should change the host vector register only when the HC
bit is clear. This will guarantee that the DSP interrupt control logic will receive a
stable vector.
5. Cancelling a Pending Host Command Exception:
The host processor may elect to clear the HC bit to cancel the host command
exception request at any time before it is recognized by the DSP. The DSP CPU
may execute the host exception after the HC bit is cleared because the host
processor does not know exactly when the exception will be recognized. This
uncertainty in timing is due to differences in synchronization between the host
processor and DSP CPU and the uncertainties of pipelined exception process-
ing. For this reason, the HV should not be changed at the same time the HC bit
is cleared. However, the HV can be changed when the HC bit is set.
6. When using the HREQ pin for handshaking, wait until HREQ is asserted and
then start writing/reading data using the HEN pin or the HACK pin.
When not using HREQ for handshaking, poll the INIT bit in the ICR to make
sure it is cleared by the hardware (which means the INIT execution is com-
pleted). Then, start writing/reading data.
If using neither HREQ for handshaking, nor polling the INIT bit, wait at least 6T
after negation of HEN that wrote ICR, before writing/reading data. This wait
ensures that the INIT is completed, because it needs 3T for synchronization
(worst case) plus 3T for executing the INIT.
7. All unused input pins should be terminated. Also, any pin that is temporarily
not driven by an output during reset, when reprogramming a port or pin, when
a bus is not driven, or at any other time, should be pulled up or down with a
resistor. For example, the HEN is capable of reacting to 2-ns noise spikes
when it is not terminated. Allowing HACK to float may cause problems even
though it is not needed in the circuit.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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