How rate of Reading SD-CARD works ?

Other Hardware Storage
1

Hello guys,

I'm using a SD-CARD with an arduino nano and uno to send images for an e-ink display.

I have to change images with high rate, and I saw that for differents images, I have differents rate of open an image from SD-CARD.

I saw that if the file is in the root of SD, I get one rate, if was in some folder I have another rate (sometimes better others worst)
The name of file is something that changes a lot the rate of function SD.open().

I tried to debug and understand why the rate changes so much. sometimes takes 100ms and others 1200ms.

Anyone can help me how to increase that rate ? the amount of files is about 600, and is a xbm image (2kb).

That is something that I can change on functions of SD librarys to change that ? ( I already set FULL_SPEEDY for the SPI)

Thankyou ,

Leonardo

2

I already set FULL_SPEEDY for the SPI

FULL_SPEEDY? That's a new one on me.

I'm pretty sure that your use of rate in your post is inaccurate, since it appears to mean duration instead.

Posting some code would be useful.

3

Sorry about my english !

For rate I mean the duration of function SD.open()

File current_image;
File next_image;

void set_spi_for_sdcard(void) {
//	SPI.end();
	SPI.begin();
	SPI.setDataMode(SPI_MODE0);
	SPI.setBitOrder(MSBFIRST);
	SPI.setClockDivider(SPI_CLOCK_DIV128);
}

void abre_imagem(const char *nome){
  
       current_image.close();
       current_image = next_image;
       next_image = SD.open(nome);
}

and the char "nome" is something like "1.144", "2.144", "3.144", etc...

I used the funciont millis() to get the duration of functin SD.open() and for each image, takes different time !

I have to increase that function !

4

The time to open a file is hard to predict. FAT directories are not ordered so open must do a linear search. Unused directory entries are scattered in the directory.

There is no simple way with SD.h to open files fast when you have 600 or more in a directory.

I would create a large contiguous file and place all the images in this file with each image in a fixed size record. I would then read the records using the raw SD read functions in SD fat.

Here is how raw read/write works. Acessing SD card without using Sd.h - #6 by fat16lib - Storage - Arduino Forum.

SdFat has a function to create a contiguous file.

bool SdBaseFile::createContiguous ( SdBaseFile * dirFile,
const char * path,
uint32_t size
)

Create and open a new contiguous file of a specified size.

Parameters:
[in] dirFile The directory where the file will be created.
[in] path A path with a valid DOS 8.3 file name.
[in] size The desired file size.

There is also a function to find the location to the file.

bool SdBaseFile::contiguousRange ( uint32_t * bgnBlock,
uint32_t * endBlock
)

Check for contiguous file and return its raw block range.

Parameters:
[out] bgnBlock the first block address for the file.
[out] endBlock the last block address for the file.