What was missing in the FatFsMemSDHC component presented here is support for a ‘write protection’ pin. Well, that write protection is not present on micro-SD cards, and on normal SD cards it is a simple plastic thing with no real hardware meaning: it is all up to the software to respect it. While my other SD card components have support for such a write protection detection, it was lacking for the FatFsMemSDHC (for Kinetis) component. Time to fix this!
SD Card Lock: an SD-Card, a micro-SD Card and a micro-SD card adapter, both with write-enabled
As you can see from above picture: the writing on the card can be very confusing about which position is the ‘lock’ (write protected).
Write Protect Pin
The component has now an optional property for a write protection pin:
Write Protect Pin in FatFsMemSDHC
I considered to replace the GPIO_LDD interface of the Card Detect Pin with the better BitIO interface (I used for the Write Protect Pin). For now I kept the existing interface for compatibility reasons. And because that would be an extra effort to change it.
Because some SD card sockets have the write protection pin with a pull-up, others have it with a pull-down, there is a ‘Low Active’ property to invert the logic.
💡 if the pin needs an internal pull-up or pull-down, you need to configure this in an extra pin Init component. See this post (section Init_GPIO) for details how to do this.
This adds a pin to the component, and the status can be checked with isWriteProtected():
Write Protection Pin Enabled
The CDE (Component Developement Code) is very simple:
bool %'ModuleName'%.%isWriteProtected(void) { %if defined(WP) %if %WPLowActive='yes' return inherited.WP.GetVal()==0; %else return inherited.WP.GetVal()!=0; %endif %else return FALSE; /* no card write protection pin available */ %endif }TWR-K60N512
I tested the function with the TWR-K60N512 board which has a SD card socket with a write protection pin:
TWR-K60N512 with SD Card
This pin is connected to PTE27:
SD_CARD_WP on TWR-K60N512 (Source: Freescale TWR-K60N512 Schematics)
Unfortunately, the schematic does not tell us if that pin will be low or high if the card is inserted :-(. For this it would be good to have a reference to the data sheet of the SD card socket used, but this was not provided (or I have not found it).
So time to make a wild guess: I assume that the WP switch is making a connection to GND, and is floating otherwise. That means that I need a pull-up, and as there is no pull-up resistor on the board schematic, I need to add an internal pull-up to the pin.
For this, I add an Init_GPIO component to the project to enable a pull-up on PTE27:
Initialization Settings for PTE27
This creates a conflict as the PTE27 is used twice. I need to enable Pin Sharing:
Enabled Pin Sharing
❗ Note: there is a bug in Processor Expert that Pin Sharing is not propagated from the ‘master’ component to the ‘slave’ component. So I need to enable Pin Sharing in the BitIO_LDD, and *not* in the BitIO component!
With this, the status of the protection tab is properly reported in the shell:
SD Card Protection Status
Summary and Sources
With this, I have added write protection pin support to the FatFsMemSDHC component. I hope this is useful for everyone.
The new component is available on GitHub.
Happy Writing 🙂
MCU on Eclipse 