High speed data transmission method for Arduino nano 33 ble

1. Tested flip pin speed of Arduino nano 33 BLE Sense

Code: 

while(1){

    digitalWrite(A0,HIGH);

    digitalWrite(A0,LOW);

}

Figure 1. approx. 426 kHz

Why so slow: Control by mbed-OS 

2. Tested SPI error at very low speed

34 became 68: 

34: 0010 0010

68: 0100 0100

Problem: clock did not align.

3. Develop corrected SPI and test its speed

FAILED: With more than 75% error transmission, it will cost at least 4 times.

Risk Evaluation:

Continuing SPI development by directly setting registers will face two risks:

• It may take many days, even weeks, to learn and debug the low-level driver for Arduino-nRF register development due to the lack of documents.
• There is still a risk of the not aligned clock problem. (might make many efforts in vain)

4. Direct operate register

NRF_P0->DIRSET = 0xFF;  //P0 lower 8bit = Input Mode

while(1){

  NRF_P0->OUTSET = 0xFF;  //P0 lower 8bit = Output High

  NRF_P0->OUTCLR = 0xFF;  //P0 lower 8bit = Output Low

}

Figure 2. Approx. 12.76MHz (Probe x10)

Even faster while using 32bits hex:

  uint32_t PortD[] = {0x00000800,0x00001000}; //D2,D3

  while(1){

    NRF_P1->OUTSET = PortD[1];

    NRF_P1->OUTCLR = PortD[1];

  }

Figure 3. Approx. 16MHz

  while(1){

      NRF_P0->OUTSET = 0xFF;  //P0 lower 8bit = Output High

      NRF_P0->OUTSET = 0xFF;

      NRF_P0->OUTSET = 0xFF;

      NRF_P0->OUTSET = 0xFF;

      NRF_P0->OUTSET = 0xFF;

      NRF_P0->OUTCLR = 0xFF;  //P0 lower 8bit = Output Low

      NRF_P0->OUTCLR = 0xFF;

      NRF_P0->OUTCLR = 0xFF;

      NRF_P0->OUTCLR = 0xFF;

      NRF_P0->OUTCLR = 0xFF;

  }

Figure 4. Approx. 4MHz (Probe x10)  under 70MHz Bandwidth Oscilloscope

Conclusion: Directly operating the register can unleash the capabilities of the nRF52840. Due to the limitations of my oscilloscope, I cannot confirm if it reaches the specifications detailed in the nRF52840 datasheet. 

5. Compare Speed

while(1){

 NRF_P0->OUTSET |= PortA[0]|PortA[1]|PortA[2]|PortA[3]|PortA[4]|PortA[5]|PortA[6]|PortA[7];

 NRF_P0->OUTCLR |= PortA[0]|PortA[1]|PortA[2]|PortA[3]|PortA[4]|PortA[5]|PortA[6]|PortA[7];

}

Result: Shifting between 4.96MHz & 6.44MHz

  while(1){

    for(int i=0; i<8; i++){

      NRF_P0->OUTSET |= PortA[i];

    }

    for(int i=0; i<8; i++){

      NRF_P0->OUTCLR |= PortA[i];

    }

  }

Result: Approx. 353kHz

  while(1){

      NRF_P0->OUTSET |= PortA[0];

      NRF_P0->OUTSET |= PortA[1];

      NRF_P0->OUTSET |= PortA[2];

      NRF_P0->OUTSET |= PortA[3];

      NRF_P0->OUTSET |= PortA[4];

      NRF_P0->OUTSET |= PortA[5];

      NRF_P0->OUTSET |= PortA[6];

      NRF_P0->OUTSET |= PortA[7];

      NRF_P0->OUTCLR |= PortA[0];

      NRF_P0->OUTCLR |= PortA[1];

      NRF_P0->OUTCLR |= PortA[2];

      NRF_P0->OUTCLR |= PortA[3];

      NRF_P0->OUTCLR |= PortA[4];

      NRF_P0->OUTCLR |= PortA[5];

      NRF_P0->OUTCLR |= PortA[6];

      NRF_P0->OUTCLR |= PortA[7];

  }

Result: Approx. 976kHz

5. Develop pallallel protocol

The IO set address is unknow and there is no information online.

The solution is to use the Arduino function:

pinMode(A1, INPUT);

And read using register:

Serial.println(NRF_P0->IN);

Basic idea：

Figure 5. The logic of communication between Machine A & Machine B

Logic:

Machine A - Step 1: Set Data Pins

Machine A - Step 2: Flip trigger-a

Machine A - Step 3: Wait() for b being flipped

Machine B - Step 2: Received a

Machine B - Step 3: Read & Save Data

Machine B - Step 4: Flip trigger-b

(Go to another loop)

This method is for the fastest transmission and confirmed reception.

Coding Design:

KEY Optimazation Method for acceration: 

• Table-based method: To calculate data in a batch and save to a array, while using it to the port, direct extract the element from the array.
• Use XOR replace if()function to acceration: NRF_P1->OUT ^= PortD[1];
• Use union

Comparison:

	double: 8byte (64bit) (ms)	float: 4byte (32bit) (ms)	int 4byte (32bit) (ms)	word: 2byte (16bit) (ms)
UART	329	229	\	175
I2C	\	\	49	33
SPI	\	\	\	\
Parallel_V0.0	302	301	\	301
Parallel_V0.1	\	\	\	\