MMC/SD power up TIME ?

[irilias]

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

i'm using an 8Mhz crystal, and i used the parameter SPI_CLK_DIV_4 for the setup_spi(), which means a 2Mhz SPi clock rate, and Tcy=1/2000000 sec (at least that's what i got from the datasheet) .anyway, for the MMC to power up it needs at least 1 msec for the supply ramp up time plus the power up time to get to VDD min (2.2 volts), my question is, how much clocks do i need to send to the MMC for it to completley power up?

my colleague told me that 80 clocks should be enough, but i don't think that would do the job, i mean :

80 x (1/2000000) is only 40 microsec ? , i probably need more than that, did i calculate the spi clock rate and Tcy correctly?

here's the code that i wrote (not all of it though, my problem is with the number of clocks to send (the FOR loop) ):

 int mmc_init() 
{ 
int i; 
SETUP_SPI(SPI_MASTER | SPI_H_TO_L | SPI_CLK_DIV_4 | SPI_SS_DISABLED); 
*0x94 |= 0x40;		// set CKE = 1 - clock idle low 
*0x14 &= 0xEF; 		// set CKP = 0 - data valid on rising edge 
OUTPUT_HIGH(PIN_C2); 	// set SS = 1 (off) 

for(i=0;i<10;i++)  	// initialise the MMC card into SPI mode by sending clks on 
{ 
SPI_WRITE(0xFF); 
} 

OUTPUT_LOW(PIN_C2);  // set SS = 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 
puts("Sent go to SPI\n\r");

any anwsers would be much appreciated, thank you.

 

[Ian Rogers]

Why worry about it.... Just wait @ 50mS just to be sure.. I'm sure your program won't miss out on 50mS...

 

[irilias]

thank you very much for your reply, well the thing is that i'm not sure if i'm doing the right calculations , i mean does the SPI_CLK_DIV_4 mean dividing 8Mhz (the crystal frequency) by 4 to get 2 Mhz the SPI clock rate, and a Tcycle of 1/2000000 sec ?

and does one Tcycle represent one clock?

i just need to clarify this thing so i can code it correctly.
Thanks again,

 

[Ian Rogers]

A pic16 with an 8mhz Xtal will be running at 2mhz... There is an internal divider BEFORE anything else... TOSC = FOSC/4..

So you need concern yourself with a clock cycle of 0.5μS... SPI, however, is good for 2mhz operation ( device dependant of course )...

I think your friend is telling you that the MMC/SD card NEEDS 80 clocks to initialize.. I have read this somewhere, I don't think its a power up time, Its a "clear out the old data" type of initialization...


I know when you try to initialize the card there will be quite a few re-tries before its up and running..

If you look at the documentation for the MMC/SD operation you can see for yourself.

 

[irilias]

ok, so the pic is running at 2Mhz, which means the SPI is functionning on 500Khz clock rate (SPI_CLK_DIV_4) . and that would give us Tcycle=1/500000 sec (equivalent of one clock), right?

if i'm on the right path, 80 clocks would still not give us the right amount of time needed for the MMC?
80 x (1/500000) = 160 microsec ?!

I'm sorry for being such a noob but i just realy want to know whether or not my reasoning is right, Thank you

 

[Ian Rogers]

No! The SD card needs 80 clocks ( physical clocks, not time...) ie.. send 0x00, 10 times with the latency of the compiler it will be fine...

 

[irilias]

ok .. that's exactly what i'm going to do,

Thank you, i really appreciate your quick answer