
CHAPTER 15 AC97U (AC97 UNIT)
User’s Manual U14579EJ2V0UM
401
15.12 Filter RAM
To use the filter function with DMA transfers, the coefficients for the calculation must be set in advance in the filter
RAM.
The method of setting this RAM is described below.
The RAM has two blocks. One is for data output (DAC) and the other is for data input (ADC).
One block is 16 bits
× 32 levels.
(1) Method of setting the data output (DAC) RAM
<1> Confirm that the DAC1_ENABLE and DAC1_STATUS bits of the DAC1_CTRL register, the
DAC2_ENABLE and DAC2_STATUS bits of the DAC2_CTRL register, and the DAC3_ENABLE and
DAC3_STATUS bits of the DAC3_CTRL register in the operational registers are 0.
<2> Set 0 in the SRC_RAM_ADR bit of the CTRL register in the operational registers.
<3> Write data 32 times in the SRC_RAM_DATA register in the operational registers (internally, the address is
automatically incremented and the data is expanded in the 32-level register).
(2) Method of setting the data input (ADC) RAM
<1> Confirm that the ADC1_ENABLE bit of the ADC1_CTRL register, the ADC2_ENABLE bit of the
ADC2_CTRL register, and the ADC3_ENABLE bit of the ADC3_CTRL register in the operational
registers are 0.
<2> Set 1 in the SRC_RAM_ADR bit of the CTRL register in the operational registers.
<3> Write data 32 times in the SRC_RAM_DATA register in the operational registers (internally, the address is
automatically incremented and the data is expanded in the 32-level register).
(3) RAM data format
The RAM data format is shown below.
Bit 15 is a sign bit
The decimal point position is the same position that is used for audio data.
Figure 15-14. RAM Data Format
Bit number
0
4
5
6
7
8
11
13 12
10
14
1
2
3
15
9
Sign bit
Decimal point position A
Decimal point position B
Remark
Coefficient range when the decimal point position is A:
1 < X < 1
Coefficient range when the decimal point position is B:
32767 < X < 32767