GD32/STM32 : WS2812B using Timer + DMA

Spi not good enough for  you?

 The easy way to use WS2812B programmable RGB leds is using the SPI MOSI pin.

That works fine most of the times with a couple of gotchas :

  • The SPI divider is not very flexible on the GD32/STM32 chips, you can only divide by a power of 2. So if your APB clock is not "near"  enought the WS2812B 400 us tick you are in trouble or must use complicated schemes.
  • Using a SPI "locks" the alternate functions of ~ 4 pins. So you can't use them for PMW or any other alternate functions, even if you dont actually use them.
  • The SPI pin is the one getting killed most often on my boards. I have a few MCUs that are working fine except the spi is fried. 
  • Also the 2nd /3rd SPI are using APB1, so not ok in terms of clock.
In short, Spi0 @ 72 Mhz is ok, other setup is to be looked closely at.

What's the other option ?


The other option is to use PWM + DMA.
We program a 1.2 us PWM mode 0 on a pin.
We change the duty at every cycle to create 0s and 1s.
For example, If the PWM is using a reload value of  100, we'll use 33 for a zero and 66 for a one as compare value for the PWM.
So we'll fill a 16 bits array of 33 and 66 to create the desired waveform, and transfer a new compare value at each PWM cycle.


DMA half transfer for the win

The main idea is as follows :
  • Our internal buffer is worth 2 leds, i.e. 48 bits => 48 x 16 bits =96 bytes 
  • We program a circular DMA transfer from our internal buffer to the CHCVs timer compare register
  • We enable the "half" and "full" transfer interrupts
  • When one occurs, it means the other half is being transfered, so we can fill the current half with the next LED value
  • When the last has been written we must wait 2 dma half cycles for it to be fully transferred

Pros & cons

Pros :
  • We can use any PWM capable pin linked to a DMA capable timer
  • We lock down only one PIN alternate function
  • The state at rest of the pin is zero
  • The timing is much more flexible as we can divide the APB clock by any number between 0 and 65535
  • The translation between "flat" value and PWM value is trivial
Cons:
  • Each led is 24 bits, each bits is converted to a 16 bits PWM compare value. That means that if we precompute everything, each Led will consume 48 bytes.
  • If we do it on the fly, we only consume 96 bytes whatever the  # of leds *BUT* we have a dma interrupt every 33 us. The computation takes about  4us (~ 2 us on a G32F303 @ 72 Mhz).
  • We probably lock a full timer, it's not easy to share different channels of a given timer
Possible improvement:
Using 8 bits->16 bits dma should work most of the times, that would divide by 2 the needed amount of data when precalculated. 


Comments

Post a Comment

Popular posts from this blog

Component tester with STM32 : Part 1 ADC, Resistor

Image
I was always  fascinated by the so call "transistor testers" that identify and report pin out, features of pretty much whatever you throw at it. They are based on Atmel 328p chip, the same as arduino nano. So why not do the same thing with a STM32/Bluepill ? The STM32 is faster, has a better ADC accuracy, more memory, can do float etc... and is not more expensive. Warning : I'm doing this for fun & to learn, it might contain error & plain misunderstanding So, i watched a couple of video on the general principle on how they work. This is not a "port" but a rewrite from ~ scratch. When something is weird, i check what the original one does for comparison. Measuring resistance So first thing first :  Resistance & Capacitance. These 2 will unlock the other ones. So how do we measure resistance ? What we do is a resistor divider with a know resistance value: Measure = Vcc*(RtoTest/(RtoTest+Rknown)) As far as the ADC goes, it means ADC=...
Read more

Fixing the INA3221

Image
It 's not too complicated to change the board we have seen  previously . You 'll need to cut some tracks, i did it with a dremel and i'm not very handy, so you'll do better. Red =Cut tracks Blue = Add wire 1- You'll need to separate the bottom connector from the ground, cut the 3 connectors BOTH SIDES. 2- Remove the small red area below the middle resistor to isolate each channels from each other Last, connect right side of each connector to the now isolated pad. Mine looks like this after the operation (ugly i know): Be wary of the small track on the left connector that leads to the left side of channel3 resistor. I cut it while isolating the right pad. Now it works, we can go back to the charger.
Read more

INA3221, weird wiring

Image
Continuing my "smart" battery charger, i've finally received the INA3221 module. It is the purple one you can find on ebay for ~ 5 bucks I soldered a couple of pins to be able to connect it so that i could test it and something immediately caught my attention. It is supposed to be a high side current/voltage sensor, typically in a setup like that : So you can measure the voltage supplied to the load, and the current going through it. Now, let's have a look at the module, look at the top of the following pic, where the sense connectors are. CH3/GND/CH2/GND/CH1/GND ? Wait ? What ? Yes, all the IN- pins are connected to ground, so it is a bottom current only sensor. Additionally, the - side of the shunts are ALSO connected to the ground, no only the connectors. The actual layout is like that : That raises two problems :  The voltage is meaningless as we are only measuring Current x ShuntValue, not the supply volt...
Read more