Finite Impulse Response (FIR) Filters¶

void
riscv_fir_f32
(const riscv_fir_instance_f32 *S, const float32_t *pSrc, float32_t *pDst, uint32_t blockSize)¶

void
riscv_fir_fast_q15
(const riscv_fir_instance_q15 *S, const q15_t *pSrc, q15_t *pDst, uint32_t blockSize)¶

IAR_ONLY_LOW_OPTIMIZATION_ENTER void riscv_fir_fast_q31(const riscv_fir_instance_q31 * S, const q31_t * pSrc, q31_t * pDst, uint32_t blockSize)

void
riscv_fir_init_f32
(riscv_fir_instance_f32 *S, uint16_t numTaps, const float32_t *pCoeffs, float32_t *pState, uint32_t blockSize)¶

riscv_status
riscv_fir_init_q15
(riscv_fir_instance_q15 *S, uint16_t numTaps, const q15_t *pCoeffs, q15_t *pState, uint32_t blockSize)¶

void
riscv_fir_init_q31
(riscv_fir_instance_q31 *S, uint16_t numTaps, const q31_t *pCoeffs, q31_t *pState, uint32_t blockSize)¶

void
riscv_fir_init_q7
(riscv_fir_instance_q7 *S, uint16_t numTaps, const q7_t *pCoeffs, q7_t *pState, uint32_t blockSize)¶

void
riscv_fir_q15
(const riscv_fir_instance_q15 *S, const q15_t *pSrc, q15_t *pDst, uint32_t blockSize)¶

void
riscv_fir_q31
(const riscv_fir_instance_q31 *S, const q31_t *pSrc, q31_t *pDst, uint32_t blockSize)¶

void
riscv_fir_q7
(const riscv_fir_instance_q7 *S, const q7_t *pSrc, q7_t *pDst, uint32_t blockSize)¶

group
FIR
This set of functions implements Finite Impulse Response (FIR) filters for Q7, Q15, Q31, and floatingpoint data types. Fast versions of Q15 and Q31 are also provided. The functions operate on blocks of input and output data and each call to the function processes
blockSize
samples through the filter.pSrc
andpDst
points to input and output arrays containingblockSize
values. Algorithm
The FIR filter algorithm is based upon a sequence of multiplyaccumulate (MAC) operations. Each filter coefficient
b[n]
is multiplied by a state variable which equals a previous input samplex[n]
.pCoeffs
points to a coefficient array of sizenumTaps
. Coefficients are stored in time reversed order.pState
points to a state array of sizenumTaps + blockSize  1
. Samples in the state buffer are stored in the following order.Note that the length of the state buffer exceeds the length of the coefficient array by
blockSize1
. The increased state buffer length allows circular addressing, which is traditionally used in the FIR filters, to be avoided and yields a significant speed improvement. The state variables are updated after each block of data is processed; the coefficients are untouched. Instance Structure
The coefficients and state variables for a filter are stored together in an instance data structure. A separate instance structure must be defined for each filter. Coefficient arrays may be shared among several instances while state variable arrays cannot be shared. There are separate instance structure declarations for each of the 4 supported data types.
 Initialization Functions
There is also an associated initialization function for each data type. The initialization function performs the following operations:
Sets the values of the internal structure fields.
Zeros out the values in the state buffer. To do this manually without calling the init function, assign the follow subfields of the instance structure: numTaps, pCoeffs, pState. Also set all of the values in pState to zero.
Use of the initialization function is optional. However, if the initialization function is used, then the instance structure cannot be placed into a const data section. To place an instance structure into a const data section, the instance structure must be manually initialized. Set the values in the state buffer to zeros before static initialization. The code below statically initializes each of the 4 different data type filter instance structures where
numTaps
is the number of filter coefficients in the filter;pState
is the address of the state buffer;pCoeffs
is the address of the coefficient buffer. FixedPoint Behavior
Care must be taken when using the fixedpoint versions of the FIR filter functions. In particular, the overflow and saturation behavior of the accumulator used in each function must be considered. Refer to the function specific documentation below for usage guidelines.
Functions

void
riscv_fir_f32
(const riscv_fir_instance_f32 *S, const float32_t *pSrc, float32_t *pDst, uint32_t blockSize) Processing function for floatingpoint FIR filter.
Processing function for the floatingpoint FIR filter.
 Return
none
 Parameters
[in] S
: points to an instance of the floatingpoint FIR filter structure[in] pSrc
: points to the block of input data[out] pDst
: points to the block of output data[in] blockSize
: number of samples to process

void
riscv_fir_fast_q15
(const riscv_fir_instance_q15 *S, const q15_t *pSrc, q15_t *pDst, uint32_t blockSize) Processing function for the Q15 FIR filter (fast version).
Processing function for the fast Q15 FIR filter (fast version).
 Return
none
 Scaling and Overflow Behavior
This fast version uses a 32bit accumulator with 2.30 format. The accumulator maintains full precision of the intermediate multiplication results but provides only a single guard bit. Thus, if the accumulator result overflows it wraps around and distorts the result. In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits. The 2.30 accumulator is then truncated to 2.15 format and saturated to yield the 1.15 result.
 Remark
Refer to riscv_fir_q15() for a slower implementation of this function which uses 64bit accumulation to avoid wrap around distortion. Both the slow and the fast versions use the same instance structure. Use function riscv_fir_init_q15() to initialize the filter structure.
 Parameters
[in] S
: points to an instance of the Q15 FIR filter structure[in] pSrc
: points to the block of input data[out] pDst
: points to the block of output data[in] blockSize
: number of samples to process

IAR_ONLY_LOW_OPTIMIZATION_ENTER void riscv_fir_fast_q31(const riscv_fir_instance_q31 * S, const q31_t * pSrc, q31_t * pDst, uint32_t blockSize)
Processing function for the Q31 FIR filter (fast version).
Processing function for the fast Q31 FIR filter (fast version).
 Return
none
 Scaling and Overflow Behavior
This function is optimized for speed at the expense of fixedpoint precision and overflow protection. The result of each 1.31 x 1.31 multiplication is truncated to 2.30 format. These intermediate results are added to a 2.30 accumulator. Finally, the accumulator is saturated and converted to a 1.31 result. The fast version has the same overflow behavior as the standard version and provides less precision since it discards the low 32 bits of each multiplication result. In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits.
 Remark
Refer to riscv_fir_q31() for a slower implementation of this function which uses a 64bit accumulator to provide higher precision. Both the slow and the fast versions use the same instance structure. Use function riscv_fir_init_q31() to initialize the filter structure.
 Parameters
[in] S
: points to an instance of the Q31 structure[in] pSrc
: points to the block of input data[out] pDst
: points to the block of output data[in] blockSize
: number of samples to process

void
riscv_fir_init_f32
(riscv_fir_instance_f32 *S, uint16_t numTaps, const float32_t *pCoeffs, float32_t *pState, uint32_t blockSize) Initialization function for the floatingpoint FIR filter.
 Return
none
 Details
pCoeffs
points to the array of filter coefficients stored in time reversed order:pState
points to the array of state variables.pState
is of lengthnumTaps+blockSize1
samples, whereblockSize
is the number of input samples processed by each call toriscv_fir_f32()
. Parameters
[inout] S
: points to an instance of the floatingpoint FIR filter structure[in] numTaps
: number of filter coefficients in the filter[in] pCoeffs
: points to the filter coefficients buffer[in] pState
: points to the state buffer[in] blockSize
: number of samples processed per call

riscv_status
riscv_fir_init_q15
(riscv_fir_instance_q15 *S, uint16_t numTaps, const q15_t *pCoeffs, q15_t *pState, uint32_t blockSize) Initialization function for the Q15 FIR filter.
 Return
execution status
RISCV_MATH_SUCCESS : Operation successful
RISCV_MATH_ARGUMENT_ERROR :
numTaps
is not greater than or equal to 4 and even
 Details
pCoeffs
points to the array of filter coefficients stored in time reversed order: Note thatnumTaps
must be even and greater than or equal to 4. To implement an odd length filter simply increasenumTaps
by 1 and set the last coefficient to zero. For example, to implement a filter withnumTaps=3
and coefficients setnumTaps=4
and use the coefficients: Similarly, to implement a two point filter setnumTaps=4
and use the coefficients:pState
points to the array of state variables.pState
is of lengthnumTaps+blockSize
, when running on RISCV Core with DSP enabled and is of lengthnumTaps+blockSize1
, when running on RISCV Core without DSP whereblockSize
is the number of input samples processed by each call toriscv_fir_q15()
. Parameters
[inout] S
: points to an instance of the Q15 FIR filter structure.[in] numTaps
: number of filter coefficients in the filter. Must be even and greater than or equal to 4.[in] pCoeffs
: points to the filter coefficients buffer.[in] pState
: points to the state buffer.[in] blockSize
: number of samples processed per call.

void
riscv_fir_init_q31
(riscv_fir_instance_q31 *S, uint16_t numTaps, const q31_t *pCoeffs, q31_t *pState, uint32_t blockSize) Initialization function for the Q31 FIR filter.
 Return
none
 Details
pCoeffs
points to the array of filter coefficients stored in time reversed order:pState
points to the array of state variables.pState
is of lengthnumTaps+blockSize1
samples, whereblockSize
is the number of input samples processed by each call toriscv_fir_q31()
. Parameters
[inout] S
: points to an instance of the Q31 FIR filter structure[in] numTaps
: number of filter coefficients in the filter[in] pCoeffs
: points to the filter coefficients buffer[in] pState
: points to the state buffer[in] blockSize
: number of samples processed

void
riscv_fir_init_q7
(riscv_fir_instance_q7 *S, uint16_t numTaps, const q7_t *pCoeffs, q7_t *pState, uint32_t blockSize) Initialization function for the Q7 FIR filter.
 Return
none
 Details
pCoeffs
points to the array of filter coefficients stored in time reversed order:pState
points to the array of state variables.pState
is of lengthnumTaps+blockSize1
samples, whereblockSize
is the number of input samples processed by each call toriscv_fir_q7()
. Parameters
[inout] S
: points to an instance of the Q7 FIR filter structure[in] numTaps
: number of filter coefficients in the filter[in] pCoeffs
: points to the filter coefficients buffer[in] pState
: points to the state buffer[in] blockSize
: number of samples processed

void
riscv_fir_q15
(const riscv_fir_instance_q15 *S, const q15_t *pSrc, q15_t *pDst, uint32_t blockSize) Processing function for the Q15 FIR filter.
 Return
none
 Scaling and Overflow Behavior
The function is implemented using a 64bit internal accumulator. Both coefficients and state variables are represented in 1.15 format and multiplications yield a 2.30 result. The 2.30 intermediate results are accumulated in a 64bit accumulator in 34.30 format. There is no risk of internal overflow with this approach and the full precision of intermediate multiplications is preserved. After all additions have been performed, the accumulator is truncated to 34.15 format by discarding low 15 bits. Lastly, the accumulator is saturated to yield a result in 1.15 format.
 Remark
Refer to riscv_fir_fast_q15() for a faster but less precise implementation of this function.
 Parameters
[in] S
: points to an instance of the Q15 FIR filter structure[in] pSrc
: points to the block of input data[out] pDst
: points to the block of output data[in] blockSize
: number of samples to process

void
riscv_fir_q31
(const riscv_fir_instance_q31 *S, const q31_t *pSrc, q31_t *pDst, uint32_t blockSize) Processing function for Q31 FIR filter.
Processing function for the Q31 FIR filter.
 Return
none
 Scaling and Overflow Behavior
The function is implemented using an internal 64bit accumulator. The accumulator has a 2.62 format and maintains full precision of the intermediate multiplication results but provides only a single guard bit. Thus, if the accumulator result overflows it wraps around rather than clip. In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits. After all multiplyaccumulates are performed, the 2.62 accumulator is right shifted by 31 bits and saturated to 1.31 format to yield the final result.
 Remark
Refer to riscv_fir_fast_q31() for a faster but less precise implementation of this filter.
 Parameters
[in] S
: points to an instance of the Q31 FIR filter structure[in] pSrc
: points to the block of input data[out] pDst
: points to the block of output data[in] blockSize
: number of samples to process

void
riscv_fir_q7
(const riscv_fir_instance_q7 *S, const q7_t *pSrc, q7_t *pDst, uint32_t blockSize) Processing function for Q7 FIR filter.
Processing function for the Q7 FIR filter.
 Return
none
 Scaling and Overflow Behavior
The function is implemented using a 32bit internal accumulator. Both coefficients and state variables are represented in 1.7 format and multiplications yield a 2.14 result. The 2.14 intermediate results are accumulated in a 32bit accumulator in 18.14 format. There is no risk of internal overflow with this approach and the full precision of intermediate multiplications is preserved. The accumulator is converted to 18.7 format by discarding the low 7 bits. Finally, the result is truncated to 1.7 format.
 Parameters
[in] S
: points to an instance of the Q7 FIR filter structure[in] pSrc
: points to the block of input data[out] pDst
: points to the block of output data[in] blockSize
: number of samples to process