Ethernet2 (UDP) SPI transfers have a lot of dead time

Hardware Arduino Due
11

I have been unable to locate [BOARD_PIN_TO_SPI_CHANNEL] to see what it is doing

It's a macro defined in variant.h:

#define BOARD_PIN_TO_SPI_CHANNEL(x) \
    (x==BOARD_SPI_SS0 ? 0 : \
    (x==BOARD_SPI_SS1 ? 1 : \
    (x==BOARD_SPI_SS2 ? 2 : 3)))

I can get faster using the multi-byte SPI transfer, but even that leaves almost a byte-time worth of gap between bytes, which is really strange. There's a byte of buffering (a full byte transmission time get get the next byte ready); it should be able to output back-to-back, I would think.

Current test program:

#include <SPI.h>

Spi *myspi;

void setup()
{
  pinMode(13, OUTPUT);
  SPI.begin();
  Serial.begin(115200);
  myspi = SPI.spi;
}

#define DATSIZE 128

byte data[DATSIZE];

void loop() {
  Serial.println("Begin SPI test");
  SPI.beginTransaction(SPISettings(28000000, MSBFIRST, SPI_MODE0));
  digitalWrite(13, LOW);
#if 1
  for (byte i = 0; i < DATSIZE; i++)
  {
    data[i] = i;
  }
  SPI.transfer(data, sizeof(data), SPI_CONTINUE);
#else
  for (byte i = 0; i < DATSIZE; i++)
  {
    SPI.transfer (i, SPI_CONTINUE);
  }
#endif
  digitalWrite(13, HIGH);
  delay(100);
  SPI.endTransaction();
  for (int j = 0; j < 4; j++) {
    Serial.println(myspi->SPI_CSR[j], HEX);
  }
  delay(5000);

}