STM32 rise/fall shape

[Matthew K]

Hi, guys. I bought

• STM32F446RE, mbed-Enabled Development Nucleo STM32F4 MCU 32-Bit ARM® Cortex®-M4 Embedded Evaluation Board (NUCLEO-F446RE)

https://www.digikey.com/product-detail/en/stmicroelectronics/NUCLEO-F446RE/497-15882-ND/5347712

• 8MHz ±10ppm Crystal 18pF 80 Ohms HC-49/US

https://www.digikey.com/product-detail/en/txc-corporation/9B-8.000MEEJ-B/887-1233-ND/2207653

Mostly the internal clock is less accurate than the external crystal. So I'm planning to populate a 8MHz clock to the board.

• If I use the external 8MHz as a clock source, do I get an accurate timing than using the internal high frequency resonator?

• I noticed the 12-bit DAC of STM32L4 discovery board showed the above result. I tried to toggle 0V ~ 3.3V after 200usec. The waveform was not flat compared to the STM32F4 discovery board's output. Since the STM32L4's APB1 clock is faster than STM32F4's APB1 clock, I thought STM32L4's DAC would do the same job. What is the reason for STM32L4 DAC's output when operating with usec range?

-Thanks!

[S.Ma · Accepted solution]

DAC is an analog static structure. Made short for conceptually understanding it's like a resistor ladder with switches to define a voltage ratio, and an optional buffer to lower the output impedence and have a decent slew rate. The DAC itself does not need clocks, only for the controls. The clock defines how fast the DAC requested voltage update can be, or output sampling rate.

Nucleo all pins are available and not tied to any external components.

For DAC outputs (PA4 and PA5 typically) on Discovery, check for schematics and make sure nothing else is impairing the analog DAC on the board.

In the datasheet, you'll find the max slew rate values. DACs are not intended to make square waves, which push-pull digital pins will do well.

Analog outputs usually are implemented by PWMs with external RC filter, which are "cheap" way to have analog signals.

When it must be high frequency analog signal, or when the analog output must be steady when the STM32 is in clock stop mode (static), DAC then shines.