GC1115_06 Datasheet, PDF (25/82 Pages) Texas Instruments – Crest Factor Reduction Processor

GC1115
www.ti.com
SLWS144C – FEBRUARY 2005 – REVISED JUNE 2006
1. Using CANCEL_ADDR and CANCEL_DATA, write up to 768 canceler coefficients and their derivatives to
shadow RAM
2. Using the RESOURCE_MASK register, specify which canceler RAMs are to be initialized from shadow RAM
when the COEF_SYNC event occurs
3. Using the COEF_SYNC register, specify which event (SW_TRIGGER, timer, SYNC_A, SYNC_B, etc.) will
trigger the COEF_SYNC event
4. After the COEF_SYNC event occurs, the GC1115 copies 3 × CANCEL_LENGTH values from shadow RAM
to the canceler RAMs selected by the RESOURCE_MASK register. During each internal clock cycle, one
shadow RAM value is copied simultaneously to the canceler RAMs enabled by their corresponding
RESOURCE_MASK bits. The total canceler RAM copy time depends on CANCEL_LENGTH and on the
time spent waiting for the selected RAMs to become idle, not on how many canceler RAMs are enabled for
update in the RESOURCE_MASK register.
5. After the GC1115 has copied 3 × CANCEL_LENGTH values from shadow RAM to canceler RAM, the
microprocessor or DSP can optionally be interrupted, if bit 6 of INT_MASK was set prior to Step 3.
Alternately, bit 6 of INT_MAP can be polled to determine when the shadow RAM copy completes.
Once the shadow RAM has been initialized, the process of copying the shadow RAM values to the canceler
RAMs occurs very quickly. For example, assuming an IN_CLK frequency of 61.44 MHz (16.3 ns), a
GC1115-internal clock rate of 4 × 61.44 MHz (4.1 ns), and a CANCEL_LENGTH of 87, the GC1115 only
requires 3 × 87 × 4.1 ns = 1.07 µs to initialize all RESOURCE_MASK-enabled canceler RAMs, assuming all
cancelers to be updated were idle when the CANCEL_SYNC trigger occurred.
CANCEL_LENGTH and CANCEL_DELAY
In many GC1115 applications, a symmetric set of cancellation coefficients is chosen to match a symmetric
carrier configuration (i.e. the center frequencies of the carrier configuration are symmetric about DC). When
cancellation coefficients are symmetric, the delay introduced by a cancellation pulse with N coefficients is N/2, or
half the cancellation pulse length. The number of samples between the first coefficient and the largest coefficient
determines CANCEL_DELAY. Since the largest coefficient of symmetric filters is usually at the midpoint of the
filter, the delay is half the filter length.
Figure 9 illustrates a typical, symmetric FIR filter impulse response, whose largest coefficient is at tap 50, at the
midpoint of the 99-coefficient filter.
Figure 9. Typical Symmetric Cancel Pulse Impulse Response, CANCEL_DELAY = 50
It is possible, however, to generate cancellation pulses whose largest coefficient is less than
CANCEL_LENGTH/2. Such cancellation pulses are called minimum phase pulses. Minimum-phase cancellation
pulses reduce the effective delay through the GC1115. Figure 10 illustrates the impulse response of a minimum
phase cancellation pulse. Notice that the largest magnitude coefficient (tap 7) is significantly closer to tap 1 than
to the middle coefficient (tap 25). The GC1115 can use either non-minimum phase or minimum phase
cancellation pulses. If a non-minimum phase filter is being used, CANCEL_DELAY = CANCEL_LENGTH / 2. If a
minimum phase filter is being used, CANCEL_DELAY < CANCEL_LENGTH / 2. CANCEL_DELAY must be
properly initialized to compensate for the cancellation pulse delay.
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