STM32U5 LPDMA with SPI and circular buffer

Question

I have a sensor (SPI slave) that follows this process:

Master sends configuration
Master sends start command
Every millisecond (or according to configuration):
1. The slave signals that data is ready by pulling a pin low
2. The master pulls CS low
3. The master sends a clock signal and reads the MISO line while the slave transfers data (in my case, 11 bytes)
4. The master pulls CS high

I've been able to configure the LPBAM to follow this basic process without too many issues. I set up a queue in the LPBAM tool in STM32CubeIDE with a single circular SPI receive data node without any trigger. The SPI device itself is set with a trigger on EXTI4, which I've connected to the data ready pin of the sensor.

However where this falls apart is collecting the read data in RAM. I want to put the sensor readings in a circular buffer or something similar and only wake the CPU when the buffer is full but I can't figure out how to make it work. If I set the "Number of Data" on the SPI receive data node, the clock signal is just held high longer.

I tried reading through the reference manual and came up with the following. The main difference that I hoped would help is that I can set the size of the SPI transfer in SPI3's CR2 register and the size of the DMA buffer separately. My hope was that the DMA would transfer one byte per request and the SPI would send one request per byte (the FIFO is set elsewhere). However I get a DMA interrupt after only two SPI reads (and then SPI chokes up and stops, I'm guessing because I haven't cleared any of its flags):

__HAL_SPI_DISABLE(&hspi3);
// Enable RX DMA requests.
SET_BIT(SPI3->CFG1, SPI_CFG1_RXDMAEN);
// Switch to simplex receiver.
SPI3->CFG2 |= (0b10 << 17);
// Set data size.
SPI3->CR2 |= 11;
// Set trigger to EXTI4 falling edge.
SPI3->AUTOCR |= (SPI_AUTOCR_TRIGEN | SPI_AUTOCR_TRIGPOL | (4 << 16));

// Reset LPDMA channel 0.
SET_BIT(LPDMA1_Channel0->CCR, 0x2);
// Enable error interrupts and half/complete transfer interrupts.
SET_BIT(LPDMA1_Channel0->CCR, (1<<14) | (1<<12) | (1<<11) | (1<<10) | (1<<9) | (1<<8));
// DINC, everything else default.
SET_BIT(LPDMA1_Channel0->CTR1, (1<<19));
// Set request to spi3 RX, BREQ to 0.
SET_BIT(LPDMA1_Channel0->CTR2, (0<<11) | 2);
// Events on block level.
CLEAR_BIT(LPDMA1_Channel0->CTR2, (1<<31) | (1<<30));
// Set size of buffer.
SET_BIT(LPDMA1_Channel0->CBR1, 500);
// Set source to SPI3 RX register.
LPDMA1_Channel0->CSAR = (uint32_t)&SPI3->RXDR;
// Set destination to buffer.
LPDMA1_Channel0->CDAR = (uint32_t)READBUFFERNAME;

// Enable LPDMA Channel 0.
SET_BIT(LPDMA1_Channel0->CCR, 1);

// Enable SPI3.
__HAL_SPI_ENABLE(&hspi3);

I found AN5816 and the presentation "DMA: Circular buffering &
double buffering" but if I understand correctly, both of these approaches rely on circular buffer support in the ADC node itself, which is missing in the SPI node.

The best I can do at the moment is to trigger an interrupt on every SPI receive and manually check on the CPU whether CDAR is approaching the end of the buffer. If it is, restart DMA at the start of the buffer. Restarting the DMA is so slow that I can easily lose a reading though, which isn't permitted in my application.

Aside from that, I could electrically connect the data ready output from the sensor into one of the LPTIM peripherals and use the timer to trigger an interrupt when the number of pulses reaches the buffer size but that feels indirect and hacky. I'd prefer to at least count transfer complete events from the LPDMA.

I feel like this should be a common use-case but I couldn't find any useful documentation or examples for it.

Does anyone have any hints?

Filling5681 · Accepted Answer

It's dirty but I think I figured out a hack: of the 11-byte payload that I get from the SPI device, I only actually care about 4 of those bytes. That just barely fits into a "single", as the DMA describes it, which allows me to hack towards what I need using TRIGM.

I get what I need using 3 LPDMA channels:

A loop that reads 11 bytes from SPI and dumps them in a buffer in RAM upon request (activated by SPI RX request, no formal trigger configuration).
A memory-to-memory DMA that copies a "single" from the SPI buffer fed by LPDMA channel 1 and puts it into a much larger buffer. The magic that makes this work is that setting TR1 SDW and DDW to 0b10 and setting TR2 TRIGM to 0b11 results in the LPDMA copying exactly 4 bytes per transfer complete, instead of the entire BNDT.
An SPI reset procedure triggered by transaction complete from queue 1, allowing it to read on the next trigger.

This is heavily inspired by the sens_acquisition demonstration module from the firmware package Github repo which uses a similar hack.

Later I'll be setting up an interrupt from channel 2 to wake the CPU and have the GPDMA copy the buffer from SRAM4 into the other SRAMs. Eventually I'll need to write the data out to SDMMC as well and the question will be what's more power efficient: wake the SDMMC frequently so that RAM can stay asleep or power on RAM so the SDMMC can stay asleep.

If I could make a wish: LPDMA should have two "BNDT" type things: one for how many bytes to read before signalling transfer complete and one for how many bytes to read per trigger.

Also for anyone else who's new and runs into similar problems: forget Cube and the shiny GUI tools. Read the reference manual. It's huge and intimidating but where Cube is confusing and opaque, the reference manual is clear and straightforward. For example Cube doesn't tell you why there are two "trigger" values when you're setting up an SPI receive in an LPBAM queue: one is for the LPDMA LLI, the other is for SPI autonomous mode. Cube also doesn't tell you that it's possible to enable an EXTI line with neither an event nor an interrupt and you'll be able to use it to trigger autonomous peripherals without waking the CPU.

Anywho, example code for my terrible hack:

/* USER CODE BEGIN PV */
static uint8_t __attribute__((section(".lpbamSection"), aligned(4))) single_reading_buffer[12] =
		{ 0 };
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) channel0_llis[2] =
		{ 0 };
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) lp_readings_buffer[4000] =
		{ 0 };
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) channel1_llis[2] =
		{ 0 };
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) spi_cr1_disable =
		0;
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) spi_cr1_enable =
		0;
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) spi_fcr_clear =
		0;
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) spi_cr2_size =
		0;
static uint32_t __attribute__((section(".lpbamSection"), aligned(4))) spi_reset_llis[4][5] =
		{ 0 };
/* USER CODE END PV */

void sendSpiStart() {
	const uint8_t start_buf = 0x08;
	if (HAL_SPI_Transmit(&hspi3, &start_buf, 1, 499) != HAL_OK) {
		Error_Handler();
	}
}

void setupSpiForLpbam() {
	__HAL_SPI_DISABLE(&hspi3);
	// Enable pre-scaler bypass, DMA requests, 8-bit frame size.
	SPI3->CFG1 = SPI_CFG1_BPASS | SPI_CFG1_RXDMAEN | 0b111;
	// Enable SS output management, clock phase, master, simplex receiver, 4-clock SS idle.
	SPI3->CFG2 = SPI_CFG2_SSOM | SPI_CFG2_SSOE | SPI_CFG2_CPHA | SPI_CFG2_MASTER
			| SPI_CFG2_COMM_1 | SPI_CFG2_MSSI_2;
	// Set transaction size to 11-byte.
	SPI3->CR2 = 11;
	// Set trigger to EXTI4 falling edge.
	SPI3->AUTOCR =
	SPI_AUTOCR_TRIGEN | SPI_AUTOCR_TRIGPOL | SPI_AUTOCR_TRIGSEL_2;
}

// Channel 0 takes SPI readings and puts them in a temporary buffer. Should output transfer complete when the block (11 bytes) is full.
void setupLpbamChannel0() {
	// Reset LPDMA channel 0.
	LPDMA1_Channel0->CCR |= DMA_CCR_RESET;
	while (LPDMA1_Channel0->CCR & DMA_CCR_EN) {
	}

	// Enable error interrupts and high priority.
	LPDMA1_Channel0->CCR = DMA_CCR_TOIE | DMA_CCR_SUSPIE | DMA_CCR_USEIE
			| DMA_CCR_ULEIE | DMA_CCR_DTEIE | DMA_CCR_PRIO_0 | DMA_CCR_PRIO_1;
	// DINC, everything else default.
	LPDMA1_Channel0->CTR1 = DMA_CTR1_DINC;
	// SPI requests, everything else default.
	LPDMA1_Channel0->CTR2 = 2;
	// Set source to SPI3 RX register.
	LPDMA1_Channel0->CSAR = (uint32_t) &SPI3->RXDR;
	// Set up an LLI that resets the block size and destination address.
	const uint32_t channel0_llis_addr = (uint32_t) &channel0_llis;
	// LLIs use a 64K-aligned base address plus a 32-bit aligned offset from that base.
	const uint32_t channel0_llis_addr_64k_aligned_base = channel0_llis_addr
			& 0xFFFF0000;
	const uint32_t channel0_llis_addr_offset_from_base = channel0_llis_addr
			& 0x0000FFFF;
	// Set size and destination of buffer.
	LPDMA1_Channel0->CBR1 = channel0_llis[0] = 11;
	// Set the destination to the second element, not the first. Since the entire buffer is 4-byte aligned, the 3 status bytes will end up out of alignment but the four 16-bit channel readings that come after will be aligned.
	LPDMA1_Channel0->CDAR = channel0_llis[1] =
			(uint32_t) &single_reading_buffer[1];
	// Set up LLI offset and which registers to update. The 0xFFFC drops the last two bits from the offset as they're ignored.
	LPDMA1_Channel0->CLBAR = channel0_llis_addr_64k_aligned_base;
	LPDMA1_Channel0->CLLR = DMA_CLLR_UB1 | DMA_CLLR_UDA
			| (0xFFFC & channel0_llis_addr_offset_from_base);
}

// Channel 1 takes the individual readings from channel 0 and moves them to a larger buffer. When the buffer is full or half-full, it sends an interrupt to wake the CPU to do more with them.
void setupLpbamChannel1() {
	// Reset LPDMA channel 1.
	LPDMA1_Channel1->CCR |= DMA_CCR_RESET;
	while (LPDMA1_Channel1->CCR & DMA_CCR_EN) {
	}

	// Enable error interrupts, half/complete transfer interrupts and high priority.
	LPDMA1_Channel1->CCR = DMA_CCR_TOIE | DMA_CCR_SUSPIE | DMA_CCR_USEIE
			| DMA_CCR_ULEIE | DMA_CCR_DTEIE | DMA_CCR_HTIE | DMA_CCR_TCIE
			| DMA_CCR_PRIO_0 | DMA_CCR_PRIO_1;
	// DINC, source and data size 1 word (32-bit, so 2 channels at 16-bit each). The word size is a hack to let us get
	// a full logical sample but treat it as a "single" that can be triggered using TRIGM in CTR2.
	LPDMA1_Channel1->CTR1 = DMA_CTR1_DINC | DMA_CTR1_DDW_LOG2_1
			| DMA_CTR1_SDW_LOG2_1;
	// Trigger on rising edge of channel 0 transfer complete, set "software" request (hopefully memory-to-memory transfer).
	// Important: 00 in TCEM means that transfer (half) complete event happens relative to the block, not the LLI.
	// Important: 11 in TRIGM means that each trigger transfers one "single", which in this case is 4 bytes (per CTR1 SDW and DDW).
	LPDMA1_Channel1->CTR2 = DMA_CTR2_TRIGPOL_0 | (18 << DMA_CTR2_TRIGSEL_Pos)
			| DMA_CTR2_SWREQ | DMA_CTR2_TRIGM_0 | DMA_CTR2_TRIGM_1;
	// Set source to the offset of the first channel in the sample buffer, which is located after a dummy byte and the 3 status bytes.
	LPDMA1_Channel1->CSAR = (uint32_t) &single_reading_buffer[4];

	// Set up an LLI that resets the block size and destination address.
	const uint32_t llis_addr = (uint32_t) &channel1_llis;
	// LLIs use a 64K-aligned base address plus a 32-bit aligned offset from that base.
	const uint32_t llis_addr_64k_aligned_base = llis_addr & 0xFFFF0000;
	const uint32_t llis_addr_offset_from_base = llis_addr & 0x0000FFFF;
	// Set size and destination of buffer.
	LPDMA1_Channel1->CBR1 = channel1_llis[0] = sizeof(lp_readings_buffer);
	LPDMA1_Channel1->CDAR = channel1_llis[1] = (uint32_t) lp_readings_buffer;
	// Set up LLI offset and which registers to update. The 0xFFFC drops the last two bits from the offset as they're ignored.
	LPDMA1_Channel1->CLBAR = llis_addr_64k_aligned_base;
	LPDMA1_Channel1->CLLR = DMA_CLLR_UB1 | DMA_CLLR_UDA
			| (0xFFFC & llis_addr_offset_from_base);
}

// Channel 2 resets the SPI peripheral after each read so that it's ready for the next one.
void setupLpbamChannel2() {
	spi_cr1_disable = SPI3->CR1 & (~SPI_CR1_SPE);
	spi_cr1_enable = SPI3->CR1 | SPI_CR1_SPE;
	spi_fcr_clear = 0x007F00;
	spi_cr2_size = 11;

	// Reset LPDMA channel 2.
	LPDMA1_Channel2->CCR |= DMA_CCR_RESET;
	while (LPDMA1_Channel2->CCR & DMA_CCR_EN) {
	}

	// Enable error interrupts and high priority.
	LPDMA1_Channel2->CCR = DMA_CCR_TOIE | DMA_CCR_SUSPIE | DMA_CCR_USEIE
			| DMA_CCR_ULEIE | DMA_CCR_DTEIE | DMA_CCR_PRIO_0 | DMA_CCR_PRIO_1;
	// Everything default, including source/dest address increment.
	LPDMA1_Channel2->CTR1 = 0;

	// Set up an LLI that resets the block size and destination address.
	const uint32_t llis_addr = (uint32_t) &spi_reset_llis;
	// LLIs use a 64K-aligned base address plus a 32-bit aligned offset from that base.
	const uint32_t llis_addr_64k_aligned_base = llis_addr & 0xFFFF0000;
	const uint32_t llis_addr_offset_from_base = llis_addr & 0x0000FFFF;

	// ----- Disable SPI -----
	// CTR2: Trigger on rising edge of channel 0 transfer complete, set "software" request (hopefully memory-to-memory transfer).
	spi_reset_llis[0][0] = DMA_CTR2_TRIGPOL_0 | (18 << DMA_CTR2_TRIGSEL_Pos)
			| DMA_CTR2_SWREQ;
	// CBR1: Set transfer size to 4 bytes (size of SPI CR1 register)
	spi_reset_llis[0][1] = 4;
	// CSAR: Set source to prepared CR1 register.
	spi_reset_llis[0][2] = (uint32_t) &spi_cr1_disable;
	// CDAR: Set destination to SPI CR1
	spi_reset_llis[0][3] = (uint32_t) &SPI3->CR1;
	// Set the relevant registers to load and the address of the next LLI in the chain.
	spi_reset_llis[0][4] =
			DMA_CLLR_UT2 | DMA_CLLR_UB1 | DMA_CLLR_USA | DMA_CLLR_UDA
					| DMA_CLLR_ULL
					| (0xFFFC
							& (llis_addr_offset_from_base
									+ sizeof(spi_reset_llis[0])));

	// ----- Clear flags -----
	// CTR2: No trigger, memory-to-memory
	spi_reset_llis[1][0] = DMA_CTR2_SWREQ;
	// CBR1: Set transfer size to 4 bytes (size of SPI CR1 register)
	spi_reset_llis[1][1] = 4;
	// CSAR: Set source to prepared FCR register.
	spi_reset_llis[1][2] = (uint32_t) &spi_fcr_clear;
	// CDAR: Set destination to SPI FCR
	spi_reset_llis[1][3] = (uint32_t) &SPI3->IFCR;
	// Set the relevant registers to load and the address of the next LLI in the chain.
	spi_reset_llis[1][4] =
	DMA_CLLR_UT2 | DMA_CLLR_UB1 | DMA_CLLR_USA | DMA_CLLR_UDA | DMA_CLLR_ULL
			| (0xFFFC
					& (llis_addr_offset_from_base
							+ sizeof(spi_reset_llis[0]) * 2));

	// ----- Set size -----
	// CTR2: No trigger, memory-to-memory
	spi_reset_llis[2][0] = DMA_CTR2_SWREQ;
	// CBR1: Set transfer size to 4 bytes (size of SPI CR1 register)
	spi_reset_llis[2][1] = 4;
	// CSAR: Set source to prepared FCR register.
	spi_reset_llis[2][2] = (uint32_t) &spi_cr2_size;
	// CDAR: Set destination to SPI FCR
	spi_reset_llis[2][3] = (uint32_t) &SPI3->CR2;
	// Set the relevant registers to load and the address of the next LLI in the chain.
	spi_reset_llis[2][4] =
	DMA_CLLR_UT2 | DMA_CLLR_UB1 | DMA_CLLR_USA | DMA_CLLR_UDA | DMA_CLLR_ULL
			| (0xFFFC
					& (llis_addr_offset_from_base
							+ sizeof(spi_reset_llis[0]) * 3));

	// ----- Enable SPI -----
	// CTR2: No trigger, memory-to-memory
	spi_reset_llis[3][0] = DMA_CTR2_SWREQ;
	// CBR1: Set transfer size to 4 bytes (size of SPI CR1 register)
	spi_reset_llis[3][1] = 4;
	// CSAR: Set source to prepared FCR register.
	spi_reset_llis[3][2] = (uint32_t) &spi_cr1_enable;
	// CDAR: Set destination to SPI FCR
	spi_reset_llis[3][3] = (uint32_t) &SPI3->CR1;
	// Set the relevant registers to load and the address of first LLI.
	spi_reset_llis[3][4] =
	DMA_CLLR_UT2 | DMA_CLLR_UB1 | DMA_CLLR_USA | DMA_CLLR_UDA | DMA_CLLR_ULL
			| (0xFFFC & llis_addr_offset_from_base);

	// Set base address
	LPDMA1_Channel2->CLBAR = llis_addr_64k_aligned_base;

	// Set up first transfer
	LPDMA1_Channel2->CTR2 = spi_reset_llis[0][0];
	LPDMA1_Channel2->CBR1 = spi_reset_llis[0][1];
	LPDMA1_Channel2->CSAR = spi_reset_llis[0][2];
	LPDMA1_Channel2->CDAR = spi_reset_llis[0][3];
	LPDMA1_Channel2->CLLR = spi_reset_llis[0][4];
}

void runLpbamLowLevel() {
	// Allow LPDMA and SPI3 in sleep/stop modes.
	__HAL_RCC_LPDMA1_CLK_ENABLE();
	__HAL_RCC_LPDMA1_CLKAM_ENABLE();
	__HAL_RCC_LPDMA1_CLK_SLEEP_ENABLE();
	__HAL_RCC_SPI3_CLK_ENABLE();
	__HAL_RCC_SPI3_CLKAM_ENABLE();
	__HAL_RCC_SPI3_CLK_SLEEP_ENABLE();
	__HAL_RCC_SRAM4_CLK_ENABLE();
	__HAL_RCC_SRAM4_CLKAM_ENABLE();
	__HAL_RCC_SRAM4_CLK_SLEEP_ENABLE();
	// Set wake up clock to MSI (which is configured to 4MHz and already present during STOP modes when the CPU is likely to be woken).
	__HAL_RCC_WAKEUPSTOP_CLK_CONFIG(RCC_STOP_WAKEUPCLOCK_MSI);

	setupSpiForLpbam();
	setupLpbamChannel0();
	setupLpbamChannel1();
	setupLpbamChannel2();

	// Enable LPDMA Channel 2.
	LPDMA1_Channel2->CCR |= DMA_CCR_EN;
	LPDMA1_Channel1->CCR |= DMA_CCR_EN;
	// Enable LPDMA Channel 0.
	LPDMA1_Channel0->CCR |= DMA_CCR_EN;

	// Enable SPI3.
	__HAL_SPI_ENABLE(&hspi3);
}

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