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Pic 18f4550 interface SD card isn't working with lib ex_fat

 
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kideltn



Joined: 08 Jun 2010
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Pic 18f4550 interface SD card isn't working with lib ex_fat
PostPosted: Thu Jul 01, 2010 1:45 am     Reply with quote

After simulation Proteus interface with SD card but fail
Code:
http://www.ccsinfo.com/forum/viewtopic.php?t=42778

I think have problem in Proteus's simulation and decided to make a real circuit with 18f4550 interface SD card.
My SD card 64M,and formated FAT16.
My portion shematic :

After chance the pin in ex_fat.c to match the pin 18f4550,I load hex on Pic.
Have many strange phenomenon appeared,it is although I didn't attach sd card on circuit,the command line still appear "OK"(I have inserted the code
Code:
if(i==GOODEC) printf("OK\r\n");//if installation success,printf OK

at ex_fat.c to debug.
And when i attached sdcard,operated some command with it,ex:
Code:
mkdir aaa

The command line appear
Code:
Making directory '/aaa/':

and i can't operate the command line anymore.
I'm followed more topic about sdcard in CCSforum with lib ex_fat but still no answer.I hope to help from everyone.
Thanks alot !
petiot



Joined: 28 Jun 2010
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PostPosted: Thu Jul 01, 2010 8:12 am     Reply with quote

shouldn't your resistors on SDA and SCL pull up to not down?
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Thu Jul 01, 2010 8:23 am     Reply with quote

You need a level translator chip on the incoming data line from the SD card.
Given the minimum supply voltage for the 4550, is 4.2v, the voltage required for a 'high' on the SDI line, is 3.36v or more (if the supply voltage is higher). The SD card, only guarantees Voh to be the supply *0.75 (2.25v, with the 3v supply shown), the PIC is not going to see any 'high' data on this line....

Best Wishes
kideltn



Joined: 08 Jun 2010
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PostPosted: Thu Jul 01, 2010 9:07 pm     Reply with quote

Thanks for reply !
I used schematic from

and change pin to match 18f4550.
I don't understand Ttelmah,
Quote:

You need a level translator chip on the incoming data line from the SD card.
Given the minimum supply voltage for the 4550 is 4.2v, the voltage required for a 'high' on the SDI line, is 3.36v or more (if the supply voltage is higher). The SD card, only guarantees Voh to be the supply *0.75 (2.25v, with the 3v supply shown), the PIC is not going to see any 'high' data on this line....

I think in SDI line the SD card understand logic but not PIC ?
And above divider resistor is true ? (in SDI,SCK,CS, the voltage after dividing is 5/5*3 = 3 V with SD card. In SDO, the PIC will read the logic high 3.3 V of SD card).
It is my idea, I'm not sure its right, and I hope many are helping, especially those who were successful with this project.
Thanks a lot !
Ttelmah



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PostPosted: Fri Jul 02, 2010 2:10 am     Reply with quote

The problem is voltage levels.
If you use a PIC that runs off 3.3v, and run both the PIC, and the SD card off this voltage, then both are using the same logic 'levels', and will co-exist fine. What you have is a PIC running off a higher voltage, and because of this it both outputs higher voltages than the SD card can take (easily handled with resistors), and wants higher voltage _inputs_ than the SD card generates. This later is not do-able with resistors, but instead needs a level translator of some sort. The 'logic levels' change with different voltage supplies.
Now, PIC inputs, have two different voltage levels required, according to whether you use 'Schmitt' inputs, or 'TTL' inputs. The Schmitt inputs, have a Vih specification, of 0.8*Vcc. So for a 5v supply, 4v is _required_ for a high to be seen. The TTL inputs have a specification at 5v, on the 18F452, of 2v required for Vih. A lot lower. It is possible to run without buffers, by using sofwtare SPI, on pins without the Schmitt inputs. However the circuit you show is not doing this, and won't actually work reliably.
A search here, will find many dozens of past posts about this. It is not a PIC/SD card 'thing', since using a low voltage PIC solves the problem, but a general 'mixing logic families of different voltages' thing. You wouldn't expect a piece of equipment designed for 220v, to run on 110v supplies without an adapter, and the same applies when you mix supplies on a PCB....

Best Wishes
kideltn



Joined: 08 Jun 2010
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PostPosted: Fri Jul 02, 2010 7:50 am     Reply with quote

Thanks you again!
Really, my major is coder and my project involved sd card. I'm beginner in electronic and thanks for your explanation to help me a lot.
I read datasheet 18f4550 and decide change pin as figure:

I changed SDO to RB4 (RB4 has input TTL).
But I really am not confident with above figure.
Can you consult me it !
Thanks a lot !
Humberto



Joined: 08 Sep 2003
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PostPosted: Sun Jul 04, 2010 8:25 am     Reply with quote

You need to add a 3-10K pull up (to +5V) in DO line.

Humberto
asmallri



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PostPosted: Mon Jul 05, 2010 2:30 am     Reply with quote

If you go to my projects page there are operational examples demonstrating how to integrate and SD card with a 5 volt PIC with resistor dividers on SDI, CLK and CS of the SD cards. For a production project you need a level translator between the DO of the Card and SPI of the PIC but for a proof of concept you will find it will work ok without it but the SPI PIC input will be operating out of spec. You also need the pullup to 3.3 volts on the DO of the card as already discussed.

While it is technically possible to implement a software SPI interface to the SD card as you have done, it is not generally a good idea. You need to take into account the shear volume of traffic that needs to be exchanged between the PIC and the SD card just to do a directory lookup.
_________________
Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!


Last edited by asmallri on Mon Jul 05, 2010 5:56 am; edited 1 time in total
ckielstra



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PostPosted: Mon Jul 05, 2010 3:38 am     Reply with quote

Things become a lot easier when you use a PIC18 model that can run at 3.3V.
Why not use the PIC18LF4550? (L = Low voltage)
Or one of the many other PIC18 chips?
asmallri



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PostPosted: Mon Jul 05, 2010 4:08 am     Reply with quote

ckielstra wrote:
Things become a lot easier when you use a PIC18 model that can run at 3.3V.
Why not use the PIC18LF4550? (L = Low voltage)
Or one of the many other PIC18 chips?


One thing to consider if you are planning to use a file system is the RAM overhead required. I cannot talk to the CCS driver requirements (I do not know them) but here are the RAM requirements for ours:

RAM requirements: Standard Lite
File System overhead 560 560
Per File Overhead 540 38

Lite mode is a lower performance option but suited to limited RAM PICs. In general, I recommend using PICs with as much RAM as possible such as a PIC18F4620 with 3900 bytes of RAM or PIC18F46LF20 for a 3.3 volt version.

The PIC18F4550 if used with the USB gives you very little RAM for any real application if you want you application to support a FAT file system.
_________________
Regards, Andrew

http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!!
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