Cipher Modes; Operating Modes; Normal Cipher Mode; Step-By-Step Cipher Mode - Motorola DSP56305 User Manual

24-bit digital signal processor
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14.5

OPERATING MODES

There are four operating modes in the CCOP, two each for Cipher and Parity Coding
processing. The operating mode is determined by the OPM[1:0] bits, as described in on
page 14-16.
14.5.1

Cipher Modes

When OPM1 is cleared, the CCOP operates in one of the Cipher modes. In these modes
the CFSRs are configured as shown in Figure 14-2 CFSR Configuration in the Cipher
Modes on page 14-5. The Cipher modes are used to generate mask sequences for data
ciphering. When in the Cipher modes:
• The Feedback Tap register (CFBTz) specifies the position of the feedback taps in
the CFSRs.
• The Feedforward Tap register (CFFTz) specifies the position of the feedforward
taps from which the output data is derived.
• The Bit Select register (CBSRz) specifies which bits from the CFSR are selected for
use by the bitwise Majority function.
• The Mask register (CMSKz) specifies which of the bits selected by the Bit Select
register will be inverted before input to the bitwise majority function.
In the Cipher modes the bitwise majority function is enabled, and together with the
feedforward taps it determines the output data bit of each CFSR. Then the CFSRs output
bits are XORed together (with respect to OUTE[3:0] bits in CSFTB) to form the final
output data bit that goes to the output buffer.
14.5.1.1

Normal Cipher Mode

When OPM[1:0] equals 00, the CCOP operates in Normal Cipher mode. In this mode the
Cipher processing session progresses fluently, starting at the input phase then passing
through the run phase and finally ending in the output phase. After writing the input
data bits in the FIFO, the processing session begins by setting PREN according to the
specified configuration, counter settings, and control bits. At the end of the session, the
DSP56300 core is interrogated to read the generated data from the output FIFO. The
Normal Cipher mode operates concurrently with the DSP56300 core and requires
minimum CPU-time overhead.
14.5.1.2

Step-by-step Cipher Mode

When OPM[1:0] equals 01, the CCOP operates in the Step-by-step Cipher mode. In this
mode the Cipher processing session halts after each step. This mode of operation enables
MOTOROLA
DSP56305 User's Manual
CYCLIC CODE CO-PROCESSOR
Operating Modes
14-21

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