Due: SPI speed problem when using 2 interfaces

Projects Programming
1

HW: Due/R3 + 2x identical cheapo ebay SD module with 5v and 3.3v (there is a 3.3 vregulator onboard that presumably drops the 5v)
SW: IDE 1.5.1r2
Libraries:
#include <SPI.h>
#include <SD.h>

I have a very simple SD card cloner set up as follows: MISO,MOSI,SCK on ICSP header, with SS for src drive on 4, SS for dst drive on 10
Before we continue I must make it clear that it works perfectly when I init both SPI channels with SPI_HALF_SPEED, i.e. I have exactly cloned an SD card on this rig. Both ebay cheapo readers running at 5v. So the code and wiring look good, hence not including complete details. I call the src and dst A and B for convenience.

The problem starts when I try SPI_FULL_SPEED, and it gets weird. I have tried the following with 7 different SD or Mini SD cards, ranging from good quality Kingston SDHC 4 through noname 128MB and sizes of 1GB 2GB 4GB. The following issue occurs exactly the same each time, no matter which card I use in A or B:

early code:

#define SRC  4
#define DST  10

Sd2Card src;
Sd2Card dst;

With any/all of the cards in A, with B empty, the following code correctly reports the size:

Serial.println(src.init(SPI_FULL_SPEED,SRC));
Serial.println(src.cardSize());

The same is true for any/all cards in B with A empty::

Serial.println(dst.init(SPI_FULL_SPEED,DST));
Serial.println(dst.cardSize());

Also reports correct size

B U T if both A and B are populated, either/both code snippet fail with init returning zero! Remember, at SPI_HALF_SPEED, the amended code reports correctly in all circumstances / permutations

Summary: When both SD readers are populated,BOTH inits fail if they are initialised at full speed, even on two cards that both successfully init'ed at FULL_SPEED when mounted solo, in either reader!

H E L P!!! I want full speed, half speed gives me about 9MB/s which translates to a loooooooong time even for 1GB

Any ideas?

2

I recommend disabling all SPI devices before initializing any of them. In your case:

void setup() {

  // disable src SD
  pinMode(SRC, OUTPUT);
  digitalWrite(SRC, HIGH);

  // disable dst SD
  pinMode(DST, OUTPUT);
  digitalWrite(DST, HIGH);

  // Now start the SD cards

How does that do?

3

I might have found the answer...from wikipedia

In practice, cards are rarely ganged together because open-collector operation has problems at high speeds and increases power consumption. Newer versions of the SD specification recommend separate lines to each card

I will try init at a very low speed, then ramp up the clock before subsequent accesses and see how that works.

ps tried the disable first technique...no joy

4

The SPI bus is not open collector. It is tri-state.

edit: I suspect this is the problem:

HW: Due/R3 + 2x identical cheapo ebay SD module with 5v and 3.3v (there is a 3.3 vregulator onboard that presumably drops the 5v)

The cheapo ebay SD readers do not normally have a logic level converter. They mostly use a pullup resistor to the 3.3v power only. This could cause problems two ways.

  1. the resistors could put a load on the MOSI line that the Atmel IC cannot overcome quickly.
  2. the 3.3v logic inputs exposed to 5v output from the Atmel could cause the same problem.