PIC16F877A, supporting MMC/SD cards

[irilias]

hi, i'm new to PIC microcontrollers and will be using one (pic16f877a) to develop a project using an MMC/SD card.

i found a piece of code that initializes an MMC card into SPI mode, and sets the block length, and i wanted to add SD support to it. so i did my research and i ended up simply adding the ACMD41 command required for SD to initialize. my reasoning was as follows :

Set card select high
Send 80 SPI clock cycles (done by writing 0xFF 10 times)
Set card select low
Send CMD0 [0x400000000095]
R1 = 0x01 (idle)
Send CMD55 [0x770000000065] (1st part of ACMD41)
R1 = 0x01 (idle if SD, time out if MMC)
Send CMD41 [0x6900000000E5] (2nd part of ACMD41)
R1 = 0x00 (SD card initialized successfuly, otherwise time out "MMC")
Send CMD1 [0x4100000000FF]
R1 = 0x00 (MMC card initialized successfuly)
and then CMD16 for setting the block length

here's the code :

[asmallri]

DO you intend writing to the SD/MMC card?

If so, the PIC16F877, or for that matter any PIC16F, is a poor choice becasue of the SD/MMC card's requirement that writes be performed in 512 byte blocks. The PIC16F just does not have enough RAM nor the appropriate memory structure to support this. It would be simpler if you used a processor such as the PIC18F4620.

I do a lot of development work with SD/MMC based loggers and virtually all of my new developments requiring SD/MMC support are with the PIC24/dsPIC33 or PIC32 processors. Note that CCS does not currently offer a compiler for the PIC32 family.
_________________
Regards, Andrew

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

[temtronic]

First, there's no point commenting on the code before we know what is your hardware setup?

The reason I ask, is that everyone incorrectly assumes they can just wire up an '877 to an SD card.
Please report back with either a picture (hosted of another site) or good word description of the setup.

Generally speaking, any card <=2GB should be used.

hth
jay

[irilias]

[irilias]

[alan]

I see this schematic are the same as in this thread. Are it the same project?

http://www.ccsinfo.com/forum/viewtopic.php?t=50197

[temtronic]

Hardware is incorrect.

RC4, a Schmit trigger input, requires a minumum of .8*Vdd.
The specs are in the datasheet(table 1.2) and (Electrical 15.2)
For this PIC, .8*5.00 is 4.00 volts
The SD card can only output a maximum of 3 volts.

result. PIC never sees a '1' from the card.

Workable options include

1) selecting a 3volt rated PIC xxLFyyyz

2) additon of 'logic level translating' ic

3) selecting I/O pins that are TTL capable

The choice, as always, is up to you the designer !!

No amount of good software can work properly on bad hardware.

Also as pointed out earlier, serious lack of RAM ! Cheaper, newer, better PICs are available that will work.

hth
jay

[ckielstra]

There are more hardware problems:
- The 5V PIC output uses a resistor divider to go down to 3.3V, that's ok. The other way is wrong!The output of the MMC is 3.3V but now going through a resistor divider, making it even lower voltage.
- The two data lines from/to the MMC card need pull-up resistors, 10k will do. But this will only work when the voltage dividers are gone.

Do more investigations into your hardware. It can be done and many (wrong) examples can be found on the internet. I don't have the time right now to point you in the right direction so you'll have to do the searching yourself.

[irilias]

Thank you temtronic and ckielstra for your thoroughly answers.
i guess it's probably best to go with a logic level translating IC (maybe the 74 series 125) as it's the easiest and the most reliable solution, as for the pull up resistors, i did a little bit of research and i think it's essential to add one to CS (SS pin), as for the other pins adding pull up res is a good idea in case of a hot swap implementation..i'm just saying that, because i'm probably going to use permanent connections.
anyways i hope i'm doing this right ... here's the schematic

[img] http://imageshack.us/photo/my-images/826/picmeminterface.jpg/ [/img]

now my questions are :

- would you recommend any other level translating IC ?

- in this example [url] http://www.ljcv.net/picnet1/picnet1-ds0.pdf [/url], they were using 100 ohms resistors between the (SD-CS, SDO/RC5, SCK/RC3) and the logic level translating IC, what's their purpose? should i add them to the schematic?

thank you again for your replies

[temtronic]

The 100r resistors can be used to reduce any 'ringing' or noise as SPI is a fast data transfer. If you have room add them, they cannot hurt, cost just pennies and may actually help!
The 74act125 is a great solution. The last link to the PDF show a well thought out product.

You really shouldn't use the '877. Old, now 'obsolete', it has very limited RAM and other peripherals. Consider something newer, cheaper, pin compatible. I've settled on the 18F46K22 though there should be a 16F series you might choose.

hth
jay

[ckielstra]

The pull-up resistors are not only there for hot swapping, but also to have a defined situation at power-up. The PIC processor starts up with all I/O pins configured as inputs, this means that without pull-up (or pull-down) resistors the input pins of the MMC/SD cards will be floating at an undefined level. You don't want this, so that's why at least a pull-up for CS is required.

Then, there is another reason for the Data Out (D0 or DOut) to require a 10k pull-up: the MMC/SD card starts up in what is called the MMC-bus mode. In this mode the DOut pin is Open Collector driven, i.e. it can not generate an output voltage by itself and depends on a pull-up resistor. Only after you have sent the command to switch to SPI mode this pin will become actively driven and the pull-up is no longer required. But, for those first few commands to work correctly you can't leave the pull-up out!

[nailuy]

The simplest solutions are the best.
put the MMC at the 3.6V it should work up to 3.63 V (3,3V+10%)
put PIC16F877 at the 4.5V (data sheet page 125) or low 4V.
betweend MMC and PIC put resistors 1K.
so at 4.5V*0,8=3,6V for logic high as 3,6V MMC
Use this, is more simple and work well.
Best regards

[temtronic]

Best solution is to use an LF series of PIC which will work at 3 volts,doesn't require any 'interface' chips and has over 512 bytes of RAM.
Having '3 volt' PICs allows easy battery power.
Using '3 volt' PIC means no additional interface chips
MMC/SD cards transfer data in 512 byte 'chunks'.

Trying to use an '877 at the low end can have serious problems related to clock speed..which can be made worse by 'nonhuman' temperatures(little too cold, little too hot).

hth
jay

[asmallri]

[irilias]

Thank you guys for clearing that up, i'll add the 100 resistors and a 10k pull-up to the Dout pin of the MMC to ensure it starts up correctly once in MMC-bus mode. and i'll probably go for an LF series of PIC or a 5V PIC with enough RAM.

now since the hardware setup is correct, would you please consider taking a look at the code, it was originally written for MMC memory cards, and i tried adding SD support by using the ACMD41 (CMD55+CMD41) command required for it to initialize.

Set card select high
Send 80 SPI clock cycles (done by writing 0xFF 10 times)
Set card select low
Send CMD0 [0x400000000095]
R1 = 0x01 (idle)
Send CMD55 [0x770000000065] (1st part of ACMD41)
R1 = 0x01 (idle if SD, time out if MMC)
Send CMD41 [0x6900000000E5] (2nd part of ACMD41)
R1 = 0x00 (SD card initialized successfuly, otherwise time out "MMC")
Send CMD1 [0x4100000000FF]
R1 = 0x00 (MMC card initialized successfuly)
and then CMD16 for setting the block length

and i also did some other changes that i thought was necessary :

- i deleted the SPI_SS_DISABLED, because the slave select pin is only present in the slave
- the MMC initialization process has to be done with the clock at 400khz or less, so i used SPI_CLK_DIV_64 instead of SPI_CLK_DIV_4 to get
the clock down to 125khz. (i'm using an 8Mhz xtal)

here's the code :