SD Card access on different COG's

trancefreak

Hi,

I've a question with regards to accessing files on a SD Card with simpletools.h.
Is it possible, that two cogs are accessing two separate files at the same time using the sd driver of simpletools?

Or do I have to use two different SD Cards to achieve that?

To be more precise:
I need one cog to access file A on the SD card while another cog accesses file B on the same SD card.
Is that possible?

Thx,
Christian

jazzed

The propeller-gcc file-system code uses one cog and can open more than one file at a time.

trancefreak

Hi Jazzed,

Thanks very much for your reply and clarification!!


Thx,
Christian

David Betz

jazzed wrote: »

The propeller-gcc file-system code uses one cog and can open more than one file at a time.

I thought that the question was whether two COGs could access the filesystem at the same time. I don't think that the standard PropGCC filesystem support has sufficient locking to allow simultaneous access by multiple COGs. Is the Simple Library filesystem support different?

trancefreak

Hi David,

Yes, that was what I wanted to know.
I'm planning an implementation where two cogs are reading from one sd card. I need to play wav files. One file is background music and one is fx (speech, fx, ...).
So my idea was to have two cogs which read the audio content from sd card. So is this possible or do I have to use two sd cards to achieve this?

Regards,
Christian

David Betz

trancefreak wrote: »

Hi David,

Yes, that was what I wanted to know.
I'm planning an implementation where two cogs are reading from one sd card. I need to play wav files. One file is background music and one is fx (speech, fx, ...).
So my idea was to have two cogs which read the audio content from sd card. So is this possible or do I have to use two sd cards to achieve this?

Regards,
Christian

Unfortunately, there are a number of problems you could run into with this. First, as I said in my other message, I don't think the PropGCC filesystem will allow access from multiple COGs. However, if you're only reading the files then you might be able to access raw sectors on the SD card by just using a lock to synchronize access from multipel COGs. Another possible solution would be to have a single COG handle all file I/O but have it "serve" files to two other COGs that are handling your background music and fx. Then only the server COG will be accessing the filesystem.

jazzed

David Betz wrote: »

I thought that the question was whether two COGs could access the filesystem at the same time. I don't think that the standard PropGCC filesystem support has sufficient locking to allow simultaneous access by multiple COGs. Is the Simple Library filesystem support different?

I gave the most appropriate answer I could think of.

Jeff Martin

Using multiple cogs to access the same resource (an SD card in this case) would cause contention problems, like David noted. However, also as noted by Steve and David, you can open up more than one file at a time with one cog, and we recommend doing it this way and serving the data out to two different cogs (if need be).

Here's an answer from Andy whose thoughts I asked for outside of this thread:

...multiple files work. Starting with examples in SimpleIDE\Learn\Examples\Devices\Memory, only use one sd_mount call, and then multiple FILE *pointer = fopen statements with different pointer names. Then, use fwrite and fread and just make sure to use the right pointer. (Avoid fprintf and fscanf, they are memory hogs!)

Here's an example with a not recommended lack of tests. (The same can be done to the "with Tests" example, which tests to make sure the file successfully opened and all that.)

/*
  SD Minimal (Modified to handle two open files).side
  Create test.txt, write characters in, read back out, display. 
  
  [url]http://learn.parallax.com/propeller-c-simple-devices/sd-card-data[/url]
*/
#include "simpletools.h"                      // Include simpletools header   
int DO = 22, CLK = 23, DI = 24, CS = 25;      // SD card pins on Propeller BOE
int main(void)                                // main function
{
  sd_mount(DO, CLK, DI, CS);                  // Mount SD card
  FILE* fp = fopen("test.txt", "w");          // Open a file for writing
  FILE* fp2 = fopen("test2.txt", "w");        //  <-- Add
  fwrite("Testing 321...\n", 1, 15, fp);      // Add contents to the file
  fwrite("Testing 123...\n", 1, 15, fp2);      // Add contents to the file
  fclose(fp);                                 // Close the file
  fclose(fp2);                                 // Close the file
  
  char s[15];                                 // Buffer for characters
  char t[15];                                 // Buffer for characters
  fp = fopen("test.txt", "r");                // Reopen file for reading
  fp2 = fopen("test2.txt", "r");               // Reopen file for reading
  fread(s, 1, 15, fp);                        // Read 15 characters
  fread(t, 1, 15, fp2);                       // Read 15 characters
  fclose(fp);                                 // Close the file
  fclose(fp2);                                // Close the file
  print("First 15 chars in test.txt:\n");     // Display heading
  print("%s", s);                             // Display characters
  print("\n");                                // With a newline at the end
  print("First 15 chars in test2.txt:\n");    // Display heading
  print("%s", t);                             // Display characters
  print("\n");                                // With a newline at the end
}

Dave Hein

You should be able to use a lock to prevent the cogs from accessing the file system at the same time. However, it might be easier to use a single cog to act as a data server, as David Betz suggested. You probably need to do something like this anyhow to ensure that your buffers don't run empty when playing out a wave file.

trancefreak

Hi Jeff and Dave,

Thanks for your answers. Yes, I will do the sd access with one cog and the others will just get the file data from the "sd cog".
Question is, how the mechanism could look like to tell the "sd cog" to deliver some data. The cogs can only communicate through a mailbox. Is someone used to such a design, how to tell
another cog to deliver data from a file into buffer?

Maybe the mailbox itself has two buffers, which are filled by the "sd cog" and the buffers are filled by using flags to tell the "sd cog" to do it and another flag which is set to true, if the buffer is
filled.

Or is there a cleaner, nicer way to do this?

Christian