• PCM
Digital Audio
– It is the standard form of digital audio in computers, Compact
Discs, digital telephony and other digital audio applications. In a
PCM stream, the amplitude of the analog signal is sampled
regularly at uniform intervals, and each sample is quantized to
the nearest value within a range of digital steps.
– PCM streams have two basic properties that determine their
fidelity to the original analog signal: the sampling rate, the
number of times per second that samples are taken; and the bit
depth, which determines the number of possible digital values
that each sample can take.
• PDM
– In a PDM signal, specific amplitude values are not encoded into
pulses of different size as they would be in PCM.
– Pulse-width modulation (PWM) is the special case of PDM
where all the pulses corresponding to one sample are
contiguous in the digital signal.
– A PDM bitstream is encoded from an analog signal through the
process of delta-sigma modulation.
– The process of decoding a PDM signal into an analog one is
simple: one only has to pass the PDM signal through a low-pass
filter. This works because the function of a low-pass filter is
essentially to average the signal.
• PPM
– Pulse-position modulation (PPM) is a form of
signal modulation in which M message bits are
encoded by transmitting a single pulse in one of
2^M possible time-shifts.
CS43L22 Audio-DAC
• Discovey kit has on-board Audio-DAC with
integrated amplifier, the Cirrus Logic CS43L22
• CS43L22 has two ports: one port for the digital
audio signal, and one port for control signals.
• Both of these ports are serial interfaces, the
audio portion a fairly standard I2S interface,
the control port a standard I2C interface.
What is I2S?
• Optimized for digital audio data transmission
– 3-line serial bus lines
• SD: Two time-multiplexed data channels
• WS: Word Select (0=left channel, 1 = right channel)
• SCK: Clock
– Bus master generates SCK and WS
– Bus master = transmitter or separate controller
– SCK synchronizes transmitter and receiver
• Serial data format
• Two’s complement, MSB sent first
– If system word > #transmitted bits, truncate after LSB
– If system word < #transmitted bits, add 0’s after LSB
I2S System Conf. / Timing
Master CLK
• Looking at the schematic for the STM32F4Discovery, the I2S lines connect to pins of the
SPI3 peripheral, and the I2C lines to pins of
the I2C1 peripheral. Therefore both of these
peripherals need to be configured.
• First everything should be set up on the
STM32F4 side, then CS43L22 also needs a bit
of initializing via the control port.
Step1: Clocks
• The GPIO peripherals that we need to configure are
GPIOA (I2S_WS signal), GPIOB (I2C_SDA & I2S_SCL),
GPIOC (I2S_MCK, I2S_SCK, I2S_SD) and GPIOD (Reset
pin on CS43L22).
– RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA | RCC_AHB1Periph_GPIOB |
RCC_AHB1Periph_GPIOC | RCC_AHB1Periph_GPIOD, ENABLE)
• the two serial peripherals that we need (SPI3, I2C1they share the same system bus:APB1).
– RCC_APB1PeriphClockCmd(RCC_APB1Periph_I2C1 | RCC_APB1Periph_SPI3, ENABLE);
• the I2S peripherals have their own PLL module to
provide accurate standard audio sampling frequencies.
– RCC_PLLI2SCmd(ENABLE);
Step2: GPIOs
Step3: I2S
• we will need to #include “stm32f4xx_spi.h
I2S_Cmd(SPI3, ENABLE);
Step4: I2C
We will need to #include “stm32f4xx_i2c.h”
GPIO_SetBits(GPIOD, GPIO_Pin_4);
void send_codec_ctrl(uint8_t controlBytes[], uint8_t numBytes)
{
uint8_t bytesSent=0;
while (I2C_GetFlagStatus(CODEC_I2C, I2C_FLAG_BUSY))
{/*just wait until no longer busy*/}
I2C Send
I2C_GenerateSTART(CODEC_I2C, ENABLE);
while (!I2C_GetFlagStatus(CODEC_I2C, I2C_FLAG_SB))
{/*wait for generation of start condition*/}
I2C_Send7bitAddress(CODEC_I2C, CODEC_I2C_ADDRESS, I2C_Direction_Transmitter);
while (!I2C_CheckEvent(CODEC_I2C,
I2C_EVENT_MASTER_TRANSMITTER_MODE_SELECTED))
{/*wait for end of address transmission*/}
while (bytesSent < numBytes)
{
I2C_SendData(CODEC_I2C, controlBytes[bytesSent]);
bytesSent++;
while (!I2C_CheckEvent(CODEC_I2C, I2C_EVENT_MASTER_BYTE_TRANSMITTING))
{/*wait for transmission of byte*/}
}
while(!I2C_GetFlagStatus(CODEC_I2C, I2C_FLAG_BTF))
{/*wait until it's finished sending before creating STOP*/}
uint8_t read_codec_register(uint8_t mapbyte)
{
uint8_t receivedByte = 0;
I2C
Receive
while (I2C_GetFlagStatus(CODEC_I2C, I2C_FLAG_BUSY))
{/*just wait until no longer busy*/}
I2C_GenerateSTART(CODEC_I2C, ENABLE);
while (!I2C_GetFlagStatus(CODEC_I2C, I2C_FLAG_SB))
{/*wait for generation of start condition*/}
I2C_Send7bitAddress(CODEC_I2C, CODEC_I2C_ADDRESS, I2C_Direction_Transmitter);
while (!I2C_CheckEvent(CODEC_I2C,
I2C_EVENT_MASTER_TRANSMITTER_MODE_SELECTED))
{/*wait for end of address transmission*/}
I2C_SendData(CODEC_I2C, mapbyte); //sets the transmitter address
while (!I2C_CheckEvent(CODEC_I2C, I2C_EVENT_MASTER_BYTE_TRANSMITTING))
{/*wait for transmission of byte*/r}
I2C_GenerateSTOP(CODEC_I2C, ENABLE);
while (I2C_GetFlagStatus(CODEC_I2C, I2C_FLAG_BUSY))
{/*just wait until no longer busy*/}
I2C_AcknowledgeConfig(CODEC_I2C, DISABLE);
I2C_GenerateSTART(CODEC_I2C, ENABLE);
while (!I2C_GetFlagStatus(CODEC_I2C, I2C_FLAG_SB))
{
//wait for generation of start condition
}
I2C_Send7bitAddress(CODEC_I2C, CODEC_I2C_ADDRESS, I2C_Direction_Receiver);
while (!I2C_CheckEvent(CODEC_I2C, I2C_EVENT_MASTER_RECEIVER_MODE_SELECTED))
{
//wait for end of address transmission
}
while (!I2C_CheckEvent(CODEC_I2C, I2C_EVENT_MASTER_BYTE_RECEIVED))
{
//wait until byte arrived
}
receivedByte = I2C_ReceiveData(CODEC_I2C);
I2C_GenerateSTOP(CODEC_I2C, ENABLE);
return receivedByte;
}
MP45DT02 MEMS Microphone
• The MP45DT02 MEMS microphone outputs a PDM signal,
which is a high frequency (1 to 3.25 MHz) stream of 1-bit
digital sample
• The output is acquired in blocks of N samples by using a
synchronous serial port (SPI or I2S) of the STM32
microcontroller. The microphone's PDM output is
synchronous with its input clock; therefore an STM32 SPI/
I2S peripheral generates a clock signal for the microphone.
Step1: Clocks
Step2: GPIOs