What is the maximum SPI speed for a slave Uno/clone?

Question

I have a Rapsberry Pi master sending SPI requests to an Uno Chinese clone slave : I send 16 characters to the Arduino, which answers 16 others characters. What I send is not important, it's the Arduino answer that matters.

When I set the speed on the Raspberry Pi at 1 Mbit/s with my C program, the received answer coming from the Arduino is mostly garbage (random characters), but this issue disappears with lower speeds.

So that leads me to this question : what is the maximum SPI speed for a slave Uno/clone, in order to have no errors ?

Edit : the Arduino code :

#include <SPI.h>
char receivingBuffer[5];
char sendingBuffer[5] = {'T', 'e', 's', 't', '!'};
volatile char bytes_received = 0;
void setup()
{
 SPCR |= bit(SPE);
 pinMode(MISO, OUTPUT);
 SPI.attachInterrupt();
 SPDR = sendingBuffer[bytes_received];
}
ISR(SPI_STC_vect)
{
 if (++bytes_received == 5)
 {
 bytes_received = 0;
 }
 SPDR = sendingBuffer[bytes_received];
}
void loop()
{
 //
}

Also, the C code on the RPi :

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <wiringPiSPI.h>
#define SPI_CHANNEL 0
#define SPI_SPEED 1000000 // Hz.
int main(void)
{
 wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED);
 while (1)
 {
 char buf[5] = {'T', 'e', 's', 't', '?'};
 printf("Sending : %s\n", buf);
 wiringPiSPIDataRW(SPI_CHANNEL, buf, 5);
 printf("Receiving : %s\n\n", buf);
 sleep(1);
 }
 return 1;
}

• Majenko: The theoretical maximum is 8Mbps, which is F_CPU/2. What matters though is that you react fast enough to the requests for each byte and fill the buffer fast enough. That's down to your program to determine.
• Nick Gammon: Please show your code. What you are doing in order to answer is highly relevant. If you are sending at 1 Mbits/s then the slave doesn't have many clock cycles to formulate a response.
• Thesaurus Rex: I added the Arduino code.
• Majenko: What happens if you set a transfer speed of >1Mbps and put a delay in between each byte (at the Pi side) - does it start working then?
• Thesaurus Rex: With 1.5 or 2 Mbps, I receive the first character of the expected answer, and the other characters are the sent message's ones.

Answer 1

The maximum speed for Arduino as a slave is F_CPU/4, so it's 4Mbps. When the Arduino is the master it can work at F_CPU/2, so it's 8Mbps

• Thesaurus Rex: Thank you for your answer ! Do you have any source to confirm this ?
• bouaaah: You can find this in ATmega328 datasheet. The ATmega328 is the chip on Arduino Uno.