ST20-C1 datasheet(64/205 Pages) STMICROELECTRONICS

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5.6 Scaling

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However if the bits are not all equal, then an error has occurred. If the most significant

bit (bit 47) is zero then the overall result is positive, so the error is overflow. If the most

significant bit is 1 then the o verall result is negative, so the error is underflow. The

appropriate status register bit (

overflow or underflow) is set accordingly, and the final

value is taken from Table 5.6.

5.6.6

Post-scaling and saturation -

biquad

The result of a

biquad is always short (16-bits). The saturation test is from bit 38

upwards, and the

overflow/underflo w flags are set as required. Note the initial value of

the

overflow/underflo w flags is not taken into account. If a saturation error occurs, the

appropriate

ShortMode value from Table 5.6 is used. If no error occurs, the scaling is:

• arithmetic right shift by 23 places

• truncate to low 32 bits

5.6.7

Error: load exceptional value

There are four exceptional values based on whether the result is to be delivered as a

ShortMode (Q15) or LongMode (Q31) value, and whether the error was overflow or

underflow:

Note that in all cases the final v alue is placed in a 32-bit register (Areg).

5.6.8

Performance and interrupts

biquad

The

biquad instruction takes 11 cycles to execute, assuming single cycle memory

accesses. The ST20-C1 cannot be interrupted during this period.

smacloop

smacloop of n steps takes n+4 cycles to complete, assuming single cycle memory

accesses.

The ST20-C1 cannot be interrupted during this period, which may be up to about 8

microseconds (for single cycle memory and an operating frequency of 33 Mhz). The

user may split a long multiply accumulate into a set of shorter ones passing the inter-

mediate accumulator value from one to the next. This will reduce interrupt latency, but

loses some numerical accuracy in that within a single

smacloop intermediate values

are held to 48 bits of precision, while values passed from one

smacloop to another

have at most 30 bits of precision (and are saturated).

mac_mode

Overflo w

Underflo w

ShortMode

00007FFF

FFFF8000

LongMode

7FFFFFFF

80000000

Table 5.6 Exceptional values

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