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Once again please note that low density STM32 micros do not have any built-in DAC. For this issue I used Waveshare\u2019s Port103Z (http:\/\/www.wvshare.com\/product\/Port103Z.htm<\/a>) development board featuring STM32F103ZET6 micro. This STM32 micro has two DAC channels with separate converters and packs a great deal of other hardware resources. Most high-end micros like this one comes with at least two DAC channels. Two DAC channels are what is needed for stereo audio generation \u2013 a very common DAC-based application. The development board is cleverly designed as such that the DAC outputs are connected to LEDs for directly visualizing the DAC outputs.<\/p>\n The DAC of STM32 micros can be operated in a number of ways. As stated we can use it in independent and dual mode. Apart from this we can also trigger these DACs with external events. If no triggering option is used, the DAC outputs are updated after one clock cycle. If any triggering option is used, the outputs are updated after 3 \u2013 4 clock cycles. The outputs of the DAC block can be independently or simultaneously updated. The DAC has a maximum resolution of 12-bits. The formula for voltage output from the DAC is as follows:<\/p>\n where n is the number of bits.<\/p>\n Unless triggering is used, operating the DAC is pretty easy. We will have to turn on the desired output channel and just write the digital data value. However when it is the other way around, lot of possibilities arise. STM32F10x reference manual states all of these possibilities in dual mode operation section. Though I will not be going to detail all of these operations, the operations are easy.<\/p>\n The outputs can be internally buffered to avoid the need for an external op-amp buffer, thereby reducing both external component count and cost. I highly recommend using internals buffers in order to ensure and maintain accuracy while avoid output loading issues.<\/p>\n The other two toys that come with the STM32 DAC block are the white noise and triangular waveform generator. At first these feature may surprize you or may even make you feel that they are unnecessary. However these stuffs come handy when we need to generate arbitrary waveforms for various applications including music, tones, etc. or inject noise in a signal for debugging. Literally we can simulate a real life noisy electronic circuit environment. The triangular waveform generator can be used to add a variable amplitude triangular waveform component to a given signal. More details here: http:\/\/www.st.com\/web\/en\/resource\/technical\/document\/application_note\/CD00259245.pdf<\/a>.<\/p>\n In STM32F103ZET6, the DAC output 1 and 2 pins are located at GPIOA PA4<\/i> and PA5<\/i> respectively. Though these pins are used as outputs when used with the DAC block, ST recommends that they should be set as analogue inputs to avoid parasitic consumption \u2013 an important point to consider.<\/p>\n In STM32 micros, we just need to deal with three registers – one for configuration, one for selecting software trigger and the other for setting the DAC outputs. We do not have any other register to deal with.<\/p>\n The DAC Control Register (DAC_CR)<\/i><\/b> is the one and only register that we will need to configure the DAC. Configuring this register allows us to enable\/disable DAC channels, set output type, enable DMA support and do the other things. Both DACs of STM32 micro are configured using this register alone.<\/p>\n The next set of registers are data registers. These are the places where we will be writing digital data that will be ultimately converted to scaled analogue outputs. The DAC data registers can accept data in three integer formats – 8-bit, 12-bit right aligned and 12-bit left aligned. There are dedicated registers for these. Checkout DAC Data Holding Registers (DAC_DHRx)<\/i><\/b>. For dual mode DAC checkout DAC_DHRD<\/i><\/b> registers. Data in DAC_DHRx<\/i><\/b> registers is then loaded to DAC_DORx <\/i><\/b>registers one or several cycles later depending on if any trigger is used or not. The DAC_DORx<\/i><\/b> registers cannot be directly written.<\/p>\n Finally there is a register called DAC software trigger register (DAC_SWTRIGR)<\/i><\/b> <\/b>for setting up software trigger.<\/p>\n Just like previously we no longer need to deal with registers as I have integrated my version of SPL with the code examples. I would still recommend readers to have a glimpse of them.<\/p>\n The diagram below outlines the GPIO pins we will be using in the code examples.<\/p>\n Please note that as with any other internal block of the STM32 micro, we need to enable the DAC block\u2019s peripheral clock first then configure desired operations by setting the correct registers and finally enable the required hardware. This is the sequence we should always maintain. Required GPIO port pins should be enabled and configured as recommended prior to setting the DAC itself. Check out the clock configuration I used in all of the examples.<\/p>\n This is the rawest demo of the STM32\u2019s DAC. No triggering is used. The DAC pins output reference-shifted sine and cosine waveforms. Since the DAC\u2019s reference source in my examples is the same as that of the supply (VDD) \u2013 3.3V, the outputs are shifted by 1.65V to cover the entire sinusoidal variation. MikroC\u2019s C Math library is used for the trigonometric function. LEDs connected with DAC output pins fade and glow smoothly, showing a smooth change in voltage across them.<\/p>\n One of the cool and exceptional feature of the STM32 DAC block is its ability to generate symmetrical triangular waveforms. This waveform can be superimposed on other slow waveforms or steady DC signals. There is no need for any wave table or equation for generating the triangular wave. An internal counter takes care of generating the waveform. We can set the amplitude or peak point of the triangle wave as per our need. In order to use triangle waveform, we need to set wave type and triggering options in the DAC_CR<\/i><\/b> register. The frequency of the triangular waveform is dependent on two things \u2013 amplitude of the waveform and the frequency of trigger. In case of software trigger, we need to consider delays inside a code. These delays include delay function calls, code execution time and other factors. So far I noticed that the formula below seems to give the approximate time period of the output waveform:<\/p>\n The 2<\/i><\/b> in the above equation is due to the fact that the DAC count starts from min or 0 to max amplitude count and then reaches min count in the same way it rose. Thus from the above formula we can deduce frequency of the triangular waveform.<\/p>\n<\/a><\/p>\n<\/a><\/p>\nKey Feature of STM32 DAC Block<\/h4>\n
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Functional Description<\/h4>\n
<\/a><\/p>\n<\/a><\/p>\nRegisters<\/h4>\n
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<\/a><\/p>\nCode Examples<\/h4>\n
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\nDAC Basic Demo<\/b><\/h6>\n#include \"DAC.h\"<\/i><\/span><\/pre>\n#include \"GPIO.h\"<\/i><\/span><\/pre>\n\u00a0<\/i><\/span><\/pre>\n#define radian_per_degree\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 0.0174532925<\/i><\/span><\/pre>\n\u00a0<\/i><\/span><\/pre>\nvoid setup();<\/i><\/span><\/pre>\nvoid GPIO_setup();<\/i><\/span><\/pre>\nvoid DAC_setup();<\/i><\/span><\/pre>\n\u00a0<\/i><\/span><\/pre>\nvoid main() <\/i><\/span><\/pre>\n{<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 signed int temp1 = 0;<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 signed int temp2 = 0;<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 unsigned int degree = 0;<\/i><\/span><\/pre>\n\u00a0<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 setup();<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 <\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 while(1)<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 {<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 for(degree = 0; degree < 360; degree++)<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 {<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 temp1 = (2047 * cos(radian_per_degree * degree));<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 temp1 = (2048 - temp1);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 DAC_DHR12R1 = ((unsigned int)temp1);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 temp2 = (2047 * sin(radian_per_degree * degree));<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 temp2 = (2048 - temp2);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 DAC_DHR12R2 = ((unsigned int)temp2);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 delay_ms(6);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 }<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 };<\/i><\/span><\/pre>\n}<\/i><\/span><\/pre>\n\u00a0<\/i><\/span><\/pre>\nvoid setup()<\/i><\/span><\/pre>\n{<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0 GPIO_setup();<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0 DAC_setup();<\/i><\/span><\/pre>\n}<\/i><\/span><\/pre>\n\u00a0<\/i><\/span><\/pre>\nvoid GPIO_setup()<\/i><\/span><\/pre>\n{<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 enable_GPIOA(true);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 pin_configure_low(GPIOA_CRL, 4, analog_input);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 pin_configure_low(GPIOA_CRL, 5, analog_input);<\/i><\/span><\/pre>\n}<\/i><\/span><\/pre>\n\u00a0<\/i><\/span><\/pre>\nvoid DAC_setup()<\/i><\/span><\/pre>\n{<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 enable_DAC(true);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 DAC_reset();<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 set_DAC1_buffer(enable);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 set_DAC2_buffer(enable);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 enable_DAC1_channel(true);<\/i><\/span><\/pre>\n\u00a0\u00a0\u00a0\u00a0 enable_DAC2_channel(true);<\/i><\/span><\/pre>\n}<\/i><\/span><\/pre>\n<\/a> <\/a> <\/a>
\nVideo Link: https:\/\/www.youtube.com\/watch?v=JW6e-oHBCPQ<\/a>.<\/p>\nTriangular Waveform Generation<\/b><\/h6>\n
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