SD card advice.

Comments

• Mike Green Posts: 23,101

  2012-07-20 07:50 edited 2012-07-20 07:50

  You are supposed to have pullups. The fact that many people may be able to 'make do' without them doesn't change that. If you're having problems, you may need them
• base2design Posts: 78

  2012-07-20 08:46 edited 2012-07-20 08:46

  Thanks Mike, I'll add them. The "FAT16..." doc from Parallax shows 10K pull-ups... can I ask a really dumb question on how the value of 10K was determined?
• Dave Hein Posts: 6,347

  2012-07-20 08:58 edited 2012-07-20 08:58

  I had a similar powerup problem, which I resolved by adding a bypass cap near the power pins of the SD card socket. I don't use pullup resistors. My understanding is that almost all SD cards made within the past few years contain internal pullups. However, it doesn't hurt to wear a belt and suspenders.
• Tracy Allen Posts: 6,667

  2012-07-20 16:05 edited 2012-07-20 16:05

  The pullups are there mainly for the benefit of the µP, so that if an SD card is not present, those pins will be pulled up to a definite level and thus limit the problems that come from floating inputs. My understanding is that the presence or absence of those pullups won't affect the operation of the card.
  
  I agree that a bypass capacitor needs to be placed across SD socket power pins. The signals on clock and data lines are quite fast, so the connecting wires cannot be too long or have characteristics that slow down the signals. I'm raising a flag next to the "old floppy cable with a SD card crammed into it".
  
  The card itself will almost always have an internal pullup present on its chip select line. Thus, your program can test that line to see if a card is present. But the SD card will hardly ever have internal pullups on the other signal lines.
• Kye Posts: 2,200

  2012-07-20 20:14 edited 2012-07-20 20:14

  The pull up resistors ensure that the idle voltage on the lines is 3.3V. This makes sure that no pins are ever in an unknown state. Additionally, pull up resistors prevent CMOS inputs on the card and the propeller chip from oscillating wildly which produces extra current draw. So, pull up resistors lower the power consumption.
  
  The value of the pull up resistor is really not important. A higher value for it reduces power consumption, while a lower value for it makes the line switch faster. 10K seems to be a good place.
  
  Since the FATEngine does not open drain the CLK or DI lines the pull up resistor value is even less important as the prop chip will actively drive these I/O lines.
  
  Thanks,