PIC+MMC Circuit?

[arrow]

Hi

I have followed a previous thread on this site about a MMC circuit. That thread refers to this web page:

http://www.captain.at/electronics/pic-mmc/

However, the MMC specifications requires pull up resistors to Vsupply to be inserted on the DataIn and DataOut lines. Is the above mentioned circuit wrong? Please can someone help me.

Thank you
arrow

[ckielstra]

[arrow]

Hi Ckielstra

Thank you for the confirmation. Is it possible for you to post your circuit please?

All the best
arrow

[arrow]

Hi Ckielstra

Also, to do level shifting from 5V to 3V (or 3.3V), is there a problem using a voltage bridge with 2 resistors say 2k and 3k?

All the best
arrow

[ckielstra]

Sorry, I can't post my schematic (company confidential).

Strange, searching for an online schematic I stumbled across http://www.cc5x.de/MMC/, here they also use a setup with pull-downs i.s.o. pull-ups.

I studied the specification again, and there it is very clear: pull-ups, not pull-downs. On the CMD line a pull-up of 4k7 to 100k max. and on the DAT line a resistor of 50k to 100k max. No series resistors are used. In my schematic I use 100k pull up resistors.

Just an extra remark, beware of the cross-connection in the SPI setup:
- SPI-out of the PIC goes to DATA_IN (pin 2) of the MMC
- SPI-in of the PIC to DATA_OUT (pin 7) of the MMC

[arrow]

Hi Ckielstra

Thank you- I think what these circuits are doing is to divide the voltage so that only about 3V comes into the MMC. (The PICS output 5V and they want to have only 3V come into the MMC).
That will explain the 1.8+3.3k resistors.
But in this case I dont see any pull up/ down resistors as in the specifications.

In your circuit do you
(a) use any additional capacitors, as is recommended in the spec sheet?
(b) Do you use a 5V PIC (F) or 3V PIC (LF)?
(c) Do you connect the PIC input/ output directly into the MMC?

Thank you once again
arrow

[ckielstra]

[PCM programmer]

I was curious about that schematic. It shows a 3.3v device driving
the hardware SPI input pin (RC4 - SDI). In the Overview section
of the 16F877 data sheet, it says that RC4 has a Schmitt Trigger input.
In the Electrical section, it says that Vih for RC4 is .7 x Vdd (min).
For 5v operation, this would be 3.5v. So, supposedly a 3.3v device
could never output a sufficiently high level to meet the Vih spec.

Since their project apparently works, I decided to test this spec.
I put a 1K trimpot on pin RC4 and created the small test program below.
It continously reads the SPI data coming in on RC4 and displays it.
I found that the actual high-level threshold is about 2.50v. That's
when the trimpot is initially set to 0v, and I slowly increase the
voltage. When I turn the voltage back down, there is a bit of hysteresis,
because it doesn't start putting out 0's again until I get to about 2.25v.
Microchip specs Vil at 1.5v max, so it's actually switching at .75v above
that. I also heated up the PIC with a heat gun and froze it with freeze
spray. The thresholds changed slightly, but not very much.

This test shows why project works, even though it violates the official
specs for input levels on RC4. But I wonder why Microchip specs
such a large margin for Vih ? It switches at 2.5v, but they want
a full 1.0v margin. Anyway, I think this shows that for a hobby project
you can get away with not using level-shifters from 3.3->5v logic on
the SDI pin. I wouldn't do it in a real product, though.

[arrow]

Hi Ckielstra

You are very helpful!
I baught the 16LF873 chip, but now I have a question about programming it.
I have a very old version of PICC compiler, and a 1997 version of MPLAB. I download teh hex file into MPLAB and since it only has the 16F873 chip I use that to setting to program the LF PIC.

I plug it into the circuit and simply try to flash a few led's with the LF chip, and I see nothing. If I put in a 5V regulator (LF chip being supplied with 5V instead of 3.3V), then the LF chip flashes my leds.
Can you please tell me what I am doing wrong?

In the new version of the MPLAB is there a PIC16LF873 selection that I have to use?

Thank you once again for all your help.
arrow

[ckielstra]

MPLAB version 7.00 has no special selection for a 16LF873, only the 16F873.

Very likely your problem with not starting at 3V is the BROWNOUT fuse, set this to NOBROWNOUT and try again.

[arrow]

Hi Ckielstra

THANK YOU! that was exactly the problem with using the LF chip.

I am going to try and interface it to the MMC card now.
I will do it directly from the PIC16LF873 to the MMC card, and I will use 2 100k pull up resistors to the 3.3V on the DataIn and DataOut lines.
I will update on any progress/ problems.

All the best
arrow

[arrow]

Hi Ckielstra

I am sorry to bug you like this, but I still cannot get the MMC to work. Can you please tell me if you see anything wrong with this piece of code? (Its from Ed Waugh code). I get a time out in the mmc_response() routine.

I am not sure if SPI is working or not.
Can you please help me?

Thank you, and
All the best
arrow

#include <16F873.H>
#fuses LP, PROTECT, PUT, NOWDT, NOBROWNOUT, NOLVP, NOCPD
#use delay(clock=4000000)

short mmc_init(){
int i;

SETUP_SPI(SPI_MASTER | SPI_H_TO_L | SPI_CLK_DIV_4);

*0x94 |= 0x40; // set CKE = 1 - clock idle low
*0x14 &= 0xEF; // set CKP = 0 - data valid on rising edge

OUTPUT_HIGH(PIN_C2); // set CS = 1 (off)

for(i=0;i<10;i++){
SPI_WRITE(0xFF);
}

OUTPUT_LOW(PIN_C2); // set CS = 0 (on) tells card to go to spi mode when it receives reset

SPI_WRITE(0x40); // send reset command
SPI_WRITE(0x00); // all the arguments are 0x00 for the reset command
SPI_WRITE(0x00);
SPI_WRITE(0x00);
SPI_WRITE(0x00);
SPI_WRITE(0x95); // precalculated checksum as we are still in MMC mode

if(mmc_response(0x01)==1) return 1;

//-----
//------
}
//==============
short mmc_response(char response){
unsigned long count = 0xFFFF; // 16bit repeat, it may be possible to shrink this to 8 bit but there is not much point

displayIt(12);
delay_ms(1500);
while(SPI_READ(0xFF) != response && --count > 0);

//***IT TIMES OUT HERE and I GET A count = 0

if(count==0)displayIt(9);
if(count==1)displayIt(8);
delay_ms(1500);

if(count==0) return 1; // loop was exited due to timeout
else return 0; // loop was exited before timeout
}
//==============
void main(void){
int i;

i = mmc_init();


//------
//------
}

[ckielstra]

Just checking: Which pins of the PIC did you connect to which pins on the MMC connector?

[arrow]

Hi Ckielstra


In a 28 pin package, I connected
*pin 13 (RC2) to CS
*pin 14 (RC3) to SCKL
*pin 15 (RC4/SDI) to DOUT on this line I have a 100k to 3.3V
*pin 16 (RC5/SDO) to DIN on this line I have a 100k to 3.3V

I am a bit confused by these two statements in the code:

*0x94 |= 0x40; // set CKE = 1 - clock idle low
*0x14 &= 0xEF; // set CKP = 0 - data valid on rising edge

Are they correct? I dont understand why the operator in these two statements are different. I also dont know how the bits are set?
The specification sheet says that the lower bits in 14h should be
0000 if I am going for SPI_CLK_DIV_4 (Fosc/4).
Is the last line reflecting this?

Thank you very much
arrow

[ckielstra]

Your pin connections look fine to me.