STM32H7A3: SPI not working if SCK disabled

[AGebr.2]

I am trying to emit data on SPI MOSI without enabling the corresponding SCK pin. This is useful for instance to control WS2812-type LEDs.

​

However when I use `HAL_SPI_Transmit_IT` to transfer 3 words, the IRQ Handler gets called 3 times (triggered by a TXP event) but then it doesn't get called the 4th time because the EOT flag in SPI_SR is never raised. In other words the transfer doesn't finish. Similarly, a transfer started with `HAL_SPI_Transmit_DMA` will also not complete.

​

When I put any of the corresponding SCK GPIO​s (that exist on the package) into SPI alternate function mode, the transfer completes normally.

​

On the STM32F722 the SPI works fine without SCK pin.

​

Is this expected behavior on the STM32H7A3? The reference manual (Figure 579) suggests some entanglement between SPI core and SCK pin but I didn't find a clear statement that SCK must be enabled.

​

Below is the code that I was using for the experiments. "Works" means that the variable `works` is incremented. "Doesn't work" means that the variable `works` remains zero and the CPU spins on line 185.

#include "main.h"
 
//// works ////
//#define SPI SPI1
//#define SCK_PORT GPIOA
//#define SCK_PIN GPIO_PIN_5
 
//// works ////
//#define SPI SPI1
//#define SCK_PORT GPIOB
//#define SCK_PIN GPIO_PIN_3
 
//// doesn't work (pin not available on package) ////
//#define SPI SPI1
//#define SCK_PORT GPIOG
//#define SCK_PIN GPIO_PIN_11
 
//// works ////
//#define SPI SPI2
//#define SCK_PORT GPIOA
//#define SCK_PIN GPIO_PIN_9
 
//// works ////
//#define SPI SPI2
//#define SCK_PORT GPIOA
//#define SCK_PIN GPIO_PIN_12
 
//// can't verify ////
//#define SPI SPI2
//#define SCK_PORT GPIOB
//#define SCK_PIN GPIO_PIN_10
 
//// works ////
//#define SPI SPI2
//#define SCK_PORT GPIOB
//#define SCK_PIN GPIO_PIN_13
 
//// doesn't work (pin not available on package) ////
//#define SPI SPI2
//#define SCK_PORT GPIOD
//#define SCK_PIN GPIO_PIN_3
 
//// doesn't work (pin not available on package) ////
//#define SPI SPI2
//#define SCK_PORT GPIOI
//#define SCK_PIN GPIO_PIN_1
 
//// works ////
//#define SPI SPI3
//#define SCK_PORT GPIOB
//#define SCK_PIN GPIO_PIN_3
 
//// works ////
//#define SPI SPI3
//#define SCK_PORT GPIOC
//#define SCK_PIN GPIO_PIN_10
 
//// doesn't work ////
//#define SPI SPI1
//#define NO_SCK
 
//// doesn't work ////
//#define SPI SPI2
//#define NO_SCK
 
//// doesn't work ////
//#define SPI SPI3
//#define NO_SCK
 
 
SPI_HandleTypeDef hspi;
 
void SystemClock_Config(void);
 
int main(void) {
 HAL_Init();
 __HAL_RCC_SYSCFG_CLK_ENABLE();
 
 RCC_OscInitTypeDef RCC_OscInitStruct = {0};
 RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};
 RCC_PeriphCLKInitTypeDef PeriphClkInitStruct = {0};
 
 HAL_PWREx_ConfigSupply(PWR_LDO_SUPPLY);
 __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE0);
 while(!__HAL_PWR_GET_FLAG(PWR_FLAG_VOSRDY)) {}
 
 RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
 RCC_OscInitStruct.HSIState = RCC_HSI_DIV1;
 RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
 RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
 RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
 RCC_OscInitStruct.PLL.PLLM = 4;
 RCC_OscInitStruct.PLL.PLLN = 35;
 RCC_OscInitStruct.PLL.PLLP = 2;
 RCC_OscInitStruct.PLL.PLLQ = 2;
 RCC_OscInitStruct.PLL.PLLR = 2;
 RCC_OscInitStruct.PLL.PLLRGE = RCC_PLL1VCIRANGE_3;
 RCC_OscInitStruct.PLL.PLLVCOSEL = RCC_PLL1VCOWIDE;
 RCC_OscInitStruct.PLL.PLLFRACN = 0;
 if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
 Error_Handler();
 }
 
 RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
 |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2
 |RCC_CLOCKTYPE_D3PCLK1|RCC_CLOCKTYPE_D1PCLK1;
 RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
 RCC_ClkInitStruct.SYSCLKDivider = RCC_SYSCLK_DIV1;
 RCC_ClkInitStruct.AHBCLKDivider = RCC_HCLK_DIV1;
 RCC_ClkInitStruct.APB3CLKDivider = RCC_APB3_DIV2;
 RCC_ClkInitStruct.APB1CLKDivider = RCC_APB1_DIV2;
 RCC_ClkInitStruct.APB2CLKDivider = RCC_APB2_DIV2;
 RCC_ClkInitStruct.APB4CLKDivider = RCC_APB4_DIV2;
 if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_7) != HAL_OK) {
 Error_Handler();
 }
 
 PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_SPI1 | RCC_PERIPHCLK_SPI2 | RCC_PERIPHCLK_SPI3;
 PeriphClkInitStruct.Spi123ClockSelection = RCC_SPI123CLKSOURCE_PLL;
 if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK) {
 Error_Handler();
 }
 
 __HAL_RCC_GPIOA_CLK_ENABLE();
 __HAL_RCC_GPIOB_CLK_ENABLE();
 __HAL_RCC_GPIOC_CLK_ENABLE();
 __HAL_RCC_GPIOD_CLK_ENABLE();
 __HAL_RCC_GPIOG_CLK_ENABLE();
 __HAL_RCC_GPIOH_CLK_ENABLE();
 __HAL_RCC_GPIOI_CLK_ENABLE();
 
 __HAL_RCC_SPI1_CLK_ENABLE();
 __HAL_RCC_SPI2_CLK_ENABLE();
 __HAL_RCC_SPI3_CLK_ENABLE();
 
 HAL_NVIC_SetPriority(SPI1_IRQn, 0, 0);
 HAL_NVIC_EnableIRQ(SPI1_IRQn);
 HAL_NVIC_SetPriority(SPI2_IRQn, 0, 0);
 HAL_NVIC_EnableIRQ(SPI2_IRQn);
 HAL_NVIC_SetPriority(SPI3_IRQn, 0, 0);
 HAL_NVIC_EnableIRQ(SPI3_IRQn);
 
 hspi.Instance = SPI;
 hspi.Init.Mode = SPI_MODE_MASTER;
 hspi.Init.Direction = SPI_DIRECTION_2LINES;
 hspi.Init.DataSize = SPI_DATASIZE_16BIT;
 hspi.Init.CLKPolarity = SPI_POLARITY_LOW;
 hspi.Init.CLKPhase = SPI_PHASE_1EDGE;
 hspi.Init.NSS = SPI_NSS_SOFT;
 hspi.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_128;
 hspi.Init.FirstBit = SPI_FIRSTBIT_MSB;
 hspi.Init.TIMode = SPI_TIMODE_DISABLE;
 hspi.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
 hspi.Init.CRCPolynomial = 0x0;
 hspi.Init.NSSPMode = SPI_NSS_PULSE_ENABLE;
 hspi.Init.NSSPolarity = SPI_NSS_POLARITY_LOW;
 hspi.Init.FifoThreshold = SPI_FIFO_THRESHOLD_01DATA;
 hspi.Init.TxCRCInitializationPattern = SPI_CRC_INITIALIZATION_ALL_ZERO_PATTERN;
 hspi.Init.RxCRCInitializationPattern = SPI_CRC_INITIALIZATION_ALL_ZERO_PATTERN;
 hspi.Init.MasterSSIdleness = SPI_MASTER_SS_IDLENESS_00CYCLE;
 hspi.Init.MasterInterDataIdleness = SPI_MASTER_INTERDATA_IDLENESS_00CYCLE;
 hspi.Init.MasterReceiverAutoSusp = SPI_MASTER_RX_AUTOSUSP_DISABLE;
 hspi.Init.MasterKeepIOState = SPI_MASTER_KEEP_IO_STATE_DISABLE;
 hspi.Init.IOSwap = SPI_IO_SWAP_DISABLE;
 if (HAL_SPI_Init(&hspi) != HAL_OK) {
 Error_Handler();
 }
 
#ifndef NO_SCK
 GPIO_InitTypeDef GPIO_InitStruct = {0};
 GPIO_InitStruct.Pin = SCK_PIN;
 GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
 GPIO_InitStruct.Pull = GPIO_NOPULL;
 GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
 GPIO_InitStruct.Alternate = (SPI == SPI1) ? GPIO_AF5_SPI1 :
 (SPI == SPI2) ? GPIO_AF5_SPI2 :
 GPIO_AF6_SPI3;
 HAL_GPIO_Init(SCK_PORT, &GPIO_InitStruct);
#endif
 
 volatile int works = 0;
 while (1) {
 uint16_t data[] = {0xffff, 0xffff, 0xffff};
 HAL_SPI_Transmit_IT(&hspi, (uint8_t*)data, 3);
 while (hspi.State != HAL_SPI_STATE_READY);
 works++;
 }
}
 
void Error_Handler(void) {
 for(;;);
}
 
void SysTick_Handler(void) {
 HAL_IncTick();
}
 
void SPI1_IRQHandler(void) {
 HAL_SPI_IRQHandler(&hspi);
}
 
void SPI2_IRQHandler(void) {
 HAL_SPI_IRQHandler(&hspi);
}
 
void SPI3_IRQHandler(void) {
 HAL_SPI_IRQHandler(&hspi);
}

​

[waclawek.jan]

In the "normal" STM32 SPI the receiver is clocked by a feedback = input from the SCK pin, so while Tx works, Rx won't without enabling the SCK pin.

I suspect this may be the underlying mechanism here, too; however, the 'H7 SPI is a beast and I don't intend to dig deep into it.

JW

[AGebr.2]

Yes in slave mode the SCK=>SPI depenency makes sense, however in master mode it seems strange.

​

For the LED control I was able to work around the issue by using I2S instead of SPI.

​

However meanwhile on a different SPI instance I observed a similar issue where the SPI stalled during what might have been a strong transient coming from our power electronics. In this state `SR->CTSIZE` was 1 and `SR->EOT` remained at 0. Given the peculiar dependency between the SCK GPIO and SPI's ability to make progress I suspect that a stray signal could have been forced into SCK and tripped up the SPI.

​

Of course we should fix our electronics to avoid such issues but it's still surprising and noteworthy. For now we recover from stall conditions by resetting the SPI entirely (`__HAL_RCC_SPI3_FORCE_RESET(); __HAL_RCC_SPI3_RELEASE_RESET();​`).

​

Perhaps an ST engineer could comment on this?