Output Phase; Cipher Mode Processing - Motorola DSP56305 User Manual

CYCLIC CODE CO-PROCESSOR
Programming Considerations
terminated immediately upon starting - if a zero is detected at once, no shift will be
performed during that run phase.
14.6.3

Output Phase

The output phase is enabled only in the Cipher modes (OPM1 = 0). The output phase is
entered immediately after the run phase completes, if the Output Counter is non-zero.
When in the output phase data input is disabled and the CFSRs are shifted without any
new data being input. Every shift causes the Output Counter to be decremented. During
the output phase the stepping function is enabled, i.e. CFSRs can be selectively disabled
from shifting according to the contents of the Step Function Table. The output data bits
(and thus the feedforward tap path and the Bitwise Majority function of the CFSRs) are
enabled in the output phase. The output data bit that goes to the data FIFO register
(CDFR) is generated by XORing all enabled output bits coming from the CFSRs
(according to OUTE[3:0] bits in CSFTB). The output phase terminates when the Output
Counter reaches zero.
In the output phase, the output data bit is generated by first executing a shift in the
enabled CFSRs and then calculating the bit's value according to the feedforward tap
path and the Associated Bitwise Majority function. This procedure is suitable for some
ciphering algorithms such as the GSM A5/1. Other algorithms (e.g. GSM A5/2), which
require the output data bit to be first calculated and then the CFSRs shifted, can be
implemented by executing one shift fewer during the run phase than specified in the
algorithm (i.e. the programmer should write (#Value – 1) to the Run Counter where
#Value is the number of shifts specified by the ciphering algorithm for the run phase).
Processing is done when the input, run, and (when enabled) output phases have
completed processing.
14.6.4

Cipher Mode Processing

Cipher mode supports implementation of algorithms for data ciphering and
de-ciphering. Cipher processing is enabled when the CCOP is programmed to operate in
one of the Cipher modes (i.e., if OPM1 = 0). The processing flow is identical in both
Cipher modes.
The only difference between the two is that Step-by-step mode (OPM[1:0] = 01) clears
PREN and returns to the Idle state after executing one shift. Every shift must then be
explicitly activated (by re-setting PREN). Step-by-step mode thus allows the
programmer more intervention during Cipher processing than Normal mode.
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