When reading USART, DMA instantaneously completes

Posted on May 15, 2018 at 13:48

Please see code below.

I'm trying to read from USART using DMA. 

Please see the last line DMA1_Channel3->CCR |= DMA_CCR_EN;

When I step over that line, 

CNDTR instantaneously resets to zero like it has finished reading, also destination buffer is populated by multiple instances of the same char that was in USART1->RDR.

It looks like DMA believes that the same char is constantly arriving into RDR and available for transfer.

I don't expect any actual USART transmission at that time.

What am I doing wrong please?

void ScheduleUsartDmaRead(){

USART1->CR1 &= ~USART_CR1_TE;

USART1->CR1 |= USART_CR1_RE;

USART1->CR1 |= USART_CR1_UE; // enable usart

DMA1_Channel3->CCR &= ~DMA_CCR_EN; // disable DMA to set registers

DMA1_Channel3->CCR &= ~DMA_CCR_DIR; // peripheral to memory

DMA1->IFCR = DMA_FLAG_GL3; // clear flags

DMA1_Channel3->CNDTR = usartBufferSize; // set buffer size

DMA1_Channel3->CPAR = (uint32_t)(&(USART1->RDR)); // USART register address

DMA1_Channel3->CMAR = (uint32_t)(usartInBuffer); // memory address

DMA1_Channel3->CCR |= DMA_CCR_EN; // start DMA

}