Posts Tagged stm32l1xx

Code to produce video signals on the STM32L Discovery

I’ve finally coded the program that produces a video signal.  There weren’t too many surprises.

One issue I had was that there was that occasionally a line would be drawn slightly to the right.  I changed the interrupt priority from 7 to 0, and disabled ChibiOS’ thread preemption. Probably only changing the interrupt will do, but I had no trouble after that.

A bigger problem is that any vertical line wriggles around on the screen a lot.  I don’t think there’s anything in my code that might cause this.  I came across a question on the STM site, which suggests that an instruction must complete before an interrupt will be triggered.  If the CPU happens to be running a long instruction, the line produced by the following interrupt will be shifted over slightly.  The Arduino TV-Out library doesn’t have this problem, even though the AVRs instructions can take different amounts of time.  I’m not sure what I can do about this – the timer should still be correct, so maybe I’d need a busy-wait loop while waiting for the timer to hit a particular value.  It looks like the TV-Out library does this.  It might need some assembler, which I don’t plan to learn right now (but maybe check this ST forum post).  But the author of the RBox suggests it’s because of the wait states.  I don’t know why there would be anything non-deterministic with wait states, unless there was caching involved, which I don’t think the STM32 has.  I suspect it’s ChibiOS running stuff during the timing interrupt, which would cause jitter.

I don’t plan to do any more work on this program, since it’s served its task of teaching me about ARM processors.  It has potential to show data its capturing or interacting with an operator.  There’s plenty of memory for a framebuffer for its its 400×288 display, It should be fairly easy to port the TVOut library to it, to add graphics and text rendering capabilities.  The advantage of the ARM chip is that because it uses DMA to write to the screen, the CPU is doing almost nothing while it’s displaying an image.  An AVR needs to work hard while a line is being drawn.

I’ve seen one project where an ARM chip produced colour signals.  The CPU didn’t have DMA though, but was faster than the STM32L.  The Freescale Freedom board looks like a good target (although ChibiOS doesn’t support it yet).  I was thinking about the way that 2D polygons are drawn, and I think it might be possible to render a number of 3D polygons with occlusion as each line is being drawn.  The unusual part of this rendering is that instead of the frame rate slowing when the CPU was busy, the vertical resolution would decrease instead as the previous line keeps getting rendered as a new line is being drawn.

Generating video signals like is is nifty, but maybe a bit pointless since there are chips around with composite output anyway.  The OLinuXino iMX233 would be ideal for this, as its CPU has a complete reference manual available.  It’s designed for running Linux, but some low level programming like I did here would provide an “instant-on” function.  The same could be done with the Raspberry Pi, but since there’s no user manual available, you’d need to rely on its limited documentation and Linux drivers.  I like the idea of porting the RTEMS operating system to the OLinuXino, since that OS provides a POSIX API and BSD networking, so porting other applications would be easier.

Here’s a video of my results.  My code is here: https://github.com/33d/stm32-video

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DMA on the STM32L Discovery

There’s one more part to my video generator – the picture data, which I want to transfer to the SPI port using DMA. This actually looks fairly straightforward, these are the available registers:

MEM2MEM I’m transferring from memory to a peripheral, so this should be off.
PL I’ll make this “very high” priority, because I want to keep the picture stable at all costs. If a program writes to the framebuffer during this DMA transfer, it will be blocked.
MSIZE I’ve set the SPI port to 8 bits so I’ll stick with that. I don’t think it will make any difference whether it’s 8 or 16.
MINC I want the memory pointer to increment during the transfer
PINC I guess this should be off, because to write SPI you keep sending data to the same memory location.
CIRC I don’t want the memory pointer to circle around.
DIR Read from memory
Interrupts I won’t need any yet, but eventually I’ll have to turn off the SPI port at the end of the transfer, otherwise I’ll get white bars down the sides of the screen.

DMA_CNDTRx contains how much data to transfer. There are 7 channels, and table 40 of the reference manual says SPI2_TX is on channel 5. This needs to be set to the number of pixels / 8, since I’ll have 8 pixels in one byte.  There’s a “auto-reload” setting somewhere which resets this counter value after a transfer; I think this happens in circular mode.

Table 40 also suggests I must use DMA1 for these transfers.

The peripheral address register should point to the SPI data register (&(SPI2->DR)), and the memory register is the start of the current line of pixels.

That’s all of the available settings!  There’s one more thing to do though: section 10.3.7 says this:

The peripheral DMA requests can be independently activated/de-activated by programming the DMA control bit in the registers of the corresponding peripheral.

I guess this is the TXDMAEN bit in the SPI_CR2 register.

Now for some code… first I’ll make some data to send:

const uint8_t image[] = { 0xAA, 0x55, 0xAA, 0x55 };

Of course later on I’ll have a lot more data…

Now to set the above settings:

  DMA1_Channel5->CCR = DMA_CCR5_PL // very high priority
          | DMA_CCR5_MINC  // memory increment mode
          | DMA_CCR5_DIR;  // read from memory, not peripheral

Section 10.3.3 has this useful bit of information:

The first transfer address is the one programmed in the DMA_CPARx/DMA_CMARx registers. During transfer operations, these registers keep the initially programmed value. The current transfer addresses (in the current internal peripheral/memory address register) are not accessible by software.

This suggests that I only need to set these at the start and shouldn’t need to touch them again.

To set these:

  DMA1_Channel5->CMAR = (uint32_t) image;       // where to read from
  DMA1_Channel5->CPAR = (uint32_t) &(SPI2->DR); // where to write to

Time to try it out… and… nothing!  Maybe there’s another clock setting for DMA, and sure enough there is:

  rccEnableAHB(RCC_AHBENR_DMA1EN, 0); // Enable DMA clock, run at SYSCLK

I still haven’t got anything, so I tried setting the source and destination registers each time before I start a DMA transfer. It looks like now I get a single transfer, but I’m trying to get a transfer on every hsync.

I poked around with the debugger, especially at 0x40026058 which is DMA5->CCR1 (I calculated the address from values in stm32l1xx.h), and noticed that the Enable flag is still set.  Maybe it has to be toggled each time?  Now I get a square wave instead of my data… I then tried decreasing my hsync timer, and decreasing the SPI speed, and I got a reasonable output.  I’m getting some nasty aliasing on my DSO Nano though, maybe I should have borrowed a faster scope!  I think I was triggering the DMA transfers too quickly, which produced that square wave.  Conveniently, I notice the SPI line is now low when it’s idle, which is the output I want.  I’m not sure why it’s gone low, but I’m not complaining.

So to sum up:

  rccEnableAHB(RCC_AHBENR_DMA1EN, 0); // Enable DMA clock, run at SYSCLK
  // Configure DMA
  DMA1_Channel5->CCR = DMA_CCR5_PL // very high priority
          | DMA_CCR5_MINC  // memory increment mode
          | DMA_CCR5_DIR;  // read from memory, not peripheral
  DMA1_Channel5->CMAR = (uint32_t) image;       // where to read from
  DMA1_Channel5->CPAR = (uint32_t) &(SPI2->DR); // where to write to
...
  SPI2->CR2 = SPI_CR2_SSOE | SPI_CR2_TXDMAEN;

then in my hsync handler:

    // Activate the DMA transfer
    DMA1_Channel5->CCR &= ~DMA_CCR5_EN;
    DMA1_Channel5->CNDTR = sizeof(image);
    DMA1_Channel5->CCR |= DMA_CCR5_EN;

I didn’t need to reset CMAR and CPAR after all.

I think that’s now demonstrated everything I need for the video signal generator! My code needs a big cleanup, and I’d like to use ChibiOS functions where I can (palSetPadMode instead of messing around with memory locations and data structures, etc).

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Picture data using SPI

I plan to use SPI to send the picture data for my video generator.

First I need to work out what speed to run the port at.  Each line goes for 52 μs, or 1664 cycles.  I could divide this by 4 for 416 pixels per line or 8 for 208 per line.  This sets the baud rate, so I shouldn’t need to divide this by 8 again to get a bytes per second speed.  It looks (from the clock registers) like SPI1 is connected to the APB2 clock, and SPI2 is connected to APB1.  I’m already running APB1 at the system clock (32MHz), so I’d like to use that if I can.  The speed is set in the CR1 register, by the BR bits, which supports dividing by 4 or 8.  I might as well use SPI2.  The datasheet says that SPI2_MOSI can only be on pin B15.  I won’t need the clock output, so I won’t configure a pin for that.

The CR1 register contains a setting for 8 or 16-bit operation.  This affects the size of the data being written.  Since I plan to use DMA I’ll leave it at 8 bits.

It turns out there are very few settings to get SPI working.  I had to stuff around a lot before I got it working though – eventually I copied the ChibiOS code, and set the SPI_CR1_CPOL, SPI_CR1_SSM, SPI_CR1_SSI and SPI_CR2_SSOE flags even though I wouldn’t have thought I need them, and it suddenly worked!

This was enough to get SPI working:

  rccEnableAPB1(RCC_APB1ENR_SPI2EN, 0); // Enable SPI2 clock, run at SYSCLK
  palSetPadMode(GPIOB, 15, PAL_MODE_ALTERNATE(5) |
                           PAL_STM32_OSPEED_HIGHEST);           /* MOSI.    */
  SPI2->CR1 = //SPI_CR1_BR_0 // divide clock by 4
          SPI_CR1_CPOL | SPI_CR1_SSM | SPI_CR1_SSI |
          SPI_CR1_BR // divide clock by 256
          | SPI_CR1_MSTR;  // master mode
  SPI2->CR2 = SPI_CR2_SSOE;
  SPI2->CR1 |= SPI_CR1_SPE; // Enable SPI

To send data, write bytes to SPI2->DR.  The output appears on PB15.  I think in the future I’ll try using palSetPadMode for configuring the pins, since it’s better than the 8 lines of code I’ve been using previously to do this.  The above code divides the clock by 256 so I could see the output on my DSO Nano, but I’ll change this to 4 later.

The next step will be using DMA to write the data to SPI instead.

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