Category Archives: Electronics

CID change on SD card update (evoplus_cid)


My recent post on how to change the CID on a Samsung Evo Plus SD card has generated some interest, but also a number of people who are having problems with it. I thought it was worth posting an update with some extra information. First off, I suspect some people who are struggling have fake cards – there are a lot out there and some of them look pretty convincing. Others have suggested different hardware / firmware revisions might be an issue – quite possible but I have no way of knowing (all my Evo Plus cards work, so I can’t can’t compare against ones that don’t). I can see no reason why different phones etc. would give different results – as long as it’s a proper SD controller (not a USB mass storage adapter) then sending the command should work just fine.

Fake cards
These are very common and if you google for fake Samsung cards you’ll find lots of info on how to spot them. A few tip I’ve picked up along the way:

  • Packaging quality – the image should be well printed, in high resolution and good bit depth on the colours (some fakes looks like they’ve been converted down to 256 colours). The gloss overlay over the printed areas should align with the printing below them, if it’s offset that’s a bad sign.
  • Packaging info – the product information should be correct and match the card. I had one fake that incorrectly stated a 32gb card was SDXC on the pack instead of SDHC, the card itself had SDHC printed on it. The correct size should also be printed on the packet. Look up the UPC from the barcode on the back and make sure that matches the product and size of your card.
  • Hologram, with scratch-to-reveal verification code. The real ones have them (recent ones at least), fakes might but probably don’t. All of mine have, but oddly enough when I tried to check one on the Samsung China website I didn’t get anywhere with the verification code, the site was in Chinese though so I might have been doing something wrong.
  • The card – lots of subtle details to check. Smooth back, not lumpy showing circuit parts beneath the surface. Black on the back, white on the edges. Slight bevel on the contact side, to help insertion. Correct info printed on the card. Correct font, especially for the capacity digits, some fakes don’t use the correct slim font. Text on the back is printed so it is read with the card contacts end pointing upward. Mine are made in the Philippines but this is probably not the only place so don’t get hung up on this.
  • Card CID – check it and compare to working ones. See below…

Card CID
An example of the CID on one of my cards: 1b 534d 3030303030 10 98625deb 0102 a1. Your card CID should be very similar. The manufacturer ID should be 1b, followed by an application/OEM ID of 534d. The product name is 3030303030 (5 x ASCII ‘0’). The product revision is 10 (1.0). The next 8 hex characters (98625deb) are the SD card serial number, yours will be different! The manufacturing date is next (0102, or 0 10 2), where the first digit is ignored, the next pair is the year in hex since 2000 and the last digit is the month in hex. So this is February (2) 2016 (2000 + 0x10). I also have March 2016 (0103) cards that work fine. Last is the checksum (a1) which will be different on your card. I doubt many of the fakes have properly set Samsung CIDs so hopefully this is an easy way to tell.

My cards / System
Samsung Evo Plus 32gb. UPC: 8806086928410. Model: MB-MC32D. Model code: MB-MC32D/CN. Purchased from this listing on AliExpress. I am not affiliated with the seller and get no referral commission from this link. I also cannot guarantee that you’ll get working or even genuine cards, but I have purchased on two occasions from this seller and the cards have been genuine and worked with evoplus_cid.

I have used evoplus_cid on a Samsung Galaxy Tab 2 (10 inch, wifi model, p5110). The tablet is running CyanogenMod 13 unofficial from here.

I’ve made a couple of updates to evoplus_cid. If you supply a full 32 digit CID (and don’t apply a serial number modifier) it will be written as is without recalculation of the checksum. This was requested by a user for cards that apparently always had a checksum of 00. Although, I’ve got a laptop that always displays 00 for the checksum when showing the CID, so I wonder now if his cards really did need that! I’ve fixed a bug when compiling on 64bit Linux that could prevent the CID being written. I’ve also fixed a bug causing the displayed CID to include some extra ‘FF’s.


Changing the CID on an SD card – Working!

(tl;dr – skip to bottom for instructions, see also update post) The CID register on SD cards is supposed to be read-only, which means it can be used to lock devices to specific SD cards, preventing the user swapping them out, which is very annoying. This can result in you being stuck with a smaller card than you’d like in a device or not being able to use a backup in case of damage.

I’ve spent a while trying to work out how to change the ID on some cards. There is surprisingly little info out there about this, when it’s easy for other devices. For example, if you want to change the supposedly read-only ID on various kinds of RFID tag you can simply buy a special version from China that allows it (often referred to as gold or magic cards). There are rumours of cheap Chinese SD cards that don’t follow the official spec and allow you to change to them – I now have  large pile of cheap Chinese SD cards on my desk, but unfortunately none of them did allow it. The spec includes a command (CMD26) for writing the CID, but it’s only supposed to work once in the factory when the card is first assigned an ID. All the cards I got seemed to honour that restriction. However, there is probably some way to unlock them…

If you’ve found this page by looking for help on this topic you’ve probably already seen Bunnie Huang’s SD card hacking presentation, unfortunately I wasn’t able to track down any APPO AX211 based cards, which I thought would be my best bet.

In that presentation was mention of vendor commands for a Samsung eMMC used as the built-in storage on an Android phone. I’m not well enough versed on the technology to know quite what the difference between eMMC and SD cards are, aside from the way SD cards are packaged of course – although that might be all there is to it. Reading around they seem work in the same way and appear the same within Linux. Then I stumbled upon SamDunk, where Sean Beaupre had managed to unlock a Samsung phone (to developer mode) by reverse engineering some of the eMMC firmware on his device and finding commands that allowed the CID to be unlocked and rewritten.

So I started buying Samsung SD cards, hoping for one with the same moviNAND core as the eMMC in their phones. I now also have a stack of Samsung SD cards on my desk, but with the last different one I found I struck gold! (Collecting Samsung cards is more expensive than cheap Chinese ones and it takes some effort to avoid getting fakes.) The Samsung EVO Plus 32GB MicroSDHC card took the commands from SamDunk and let me change the CID.

I have taken the SamDunk code and reworked it a little to make it easier to use for SD cards, but all the credit really needs to go to the original author. To use you simply need a rooted Android device with an SD card slot. Copy the evoplus_cid binary to the device and put it somewhere with a proper Linux file system (not FAT), then apply execute permission to it. Run the tool and point it to the SD card device e.g. /dev/block/mmcblk1 (you almost certainly do not want to use mmcblk0, that is likely to be the built-in storage on your device). List the /dev/block folder and make sure the device you going to use appears and disappears as you insert and remove the SD card to be sure. Supply the new CID you want to set on the command line. If you only need to change the card serial number (part of the CID) you can supply the old CID and the new serial number and the new CID will be calculated for you. I suggest you make a note of the old CID first, in case you want to set it back later. This can be found in the file like /sys/class/mmc_host/mmc1/mmc1:0001/cid, but the path may vary, and again make sure you are looking at the right card by checking it disappears when you remove the card.

You can find the code here: (compiled binary is in the lib folder). Use entirely at your own risk. Use only against genuine Samsung Evo Plus cards, anything else is unlikely to work and could be damaged. Even the right card could potentially be damaged!

To set a new completely new CID:

# Usage: ./evoplus_cid <device> <new cid>
./evoplus_cid /dev/block/mmcblk1 744a454e2f412020106c6d77470104c3

Or to just change the serial number part of the current CID:

# Usage: ./evoplus_cid <device> <currentcid> <new serial>
./evoplus_cid /dev/block/mmcblk1 1b534d30303030301098625deb0102a1 12345678

Flash write bug fix

Looks like I finally have a fix for the flash write issue, thanks to some help from Espressif. Embarrassingly it turns out to be my fault, but I’m not too proud to admit that. So what was the problem? Since SDK v1.5.1 people have been reporting intermittent failed OTA updates and I’ve been able to reproduce it easily myself. This only seemed to occur when using wifi for long sustained writes.

So what was happening? Occasionally the network receive would get a much larger packet than normal. In my latest testing they are usually around 1436 bytes, but occasionally one would arrive that was 5744 (this is the only large value I have seen myself, but presumably other values could occur). The flash is erased in 4k sectors and the rBoot code erases them as required, checking if the current data block will fit in the current sector and if not erasing the next one too. Where I went wrong was to assume that a receive would always be less than the 4k sector size, having never seen one before SDK v1.5.1 that was anywhere near that large. When one of these very large packets arrives it could span 3 sectors. The second sector would get erased correctly but the third sector would not. The flash write command would not return an error when it tried to write to the non-erased sector, so no fault was noticed at this time. Then on the next write that third sector, now being the first sector for that next write, gets erased and the new data written part way through it (where it should be).

Moral of the story. Don’t make assumptions and don’t ignore the edges cases – I’m usually pretty good at this second point, but occasionally I seem to need reminding. In this case I had thought about it and added code that detected a chunk over 4k that would need more erasing. As I didn’t think this could ever happen I merely put a comment in the if statement to say what would need to be done in that scenario, if I’d thrown an error at that point instead this problem would have been easily diagnosed. However, I did also assume that the flash write would fail if it tried to write to flash that was not erased, so I expected to see an error of some kind.

Why did it suddenly happen at v1.5.1? I don’t know that but presumably Espressif made some change in the SDK that makes this more likely to occur. While playing with some code for Pete Scargill I did manage to reproduce the problem with v1.4.0 so it wasn’t impossible for it to happen there, but I never had any reports of it there previously. I also found that the timers in Pete’s code made it more likely to happen in my testing, so I suspect the extra processing these caused was impacting the performance of the network stack and causing more packets to be bunched together and delivered as larger chunks to the application. Further testing showed RF interference could also cause the same result.

The fix is available in the rBoot GitHub repo, and has also been updated in Sming.

Flash write bug in Espressif SDK v1.5.1+

Users of rBoot have reported OTA problems when using Espressif SDK v1.5.1 which I have been able to reproduce on this version and v1.5.2. The problem appears to be in the sdk flash write functions. While most writes work fine some will occasional fails to fully write (leaving small areas of blank data (0xff)). To make matters worse no error is reported from the write function and attempts to read the flash back for verification immediately afterwards return the correct data (presumably they are serviced by the rom cache). This is very similar to the problem seen when trying to perform an OTA with insufficient power. I once had a similar report from a user using just a 100mA power supply and the problem was fixed by using a proper power supply. This makes me wonder if the new SDK causes the device to draw more power, although supplying extra power does not fix the problem in this case.

I have created a test case that doesn’t use rBoot but simply downloads a file and writes it to flash (which is of course very similar to what rBoot does) to demonstrate the problem to Espressif. I have yet to hear back from them. If you encounter this problem please add your support to my bug report. I have no reason to believe this bug will be limited to rBoot users and I assume SDK bootloader users and anyone else writing to the flash from an application (possibly only in long sustained writes) will also have this problem.

This is a good opportunity to encourage rBoot users to enable the irom checksum option, to help detect badly flashed roms at boot time. See the readme for more information.

New rBoot version allows temporary boot

I’ve just pushed an update to rBoot that allows 2-way communication between rBoot and the running user application. This is something I had though about previously, and I mentioned it in a previous post, but nobody had actually asked for it until a couple of weeks ago. The main use of this is to allow the application to request rBoot to perform a temporary boot to a different rom (i.e. not the one identified in the config, which would normally be booted). This helps to make updating safer, because you can perform an OTA and only temporarily switch to the new rom, until you are happy to update the config and make this the standard. rBoot is already safer for updating than the SDK bootloader, if you enable to irom checksum option, but this new functionality also guards against valid but buggy roms that simply don’t work properly once booted.

How you decide that your rom is good enough to switch to booting it by default is up to you of course. Perhaps if the rom is able to boot, connect to wifi and stays up for 5 minutes, that would be deemed sufficient. Another option would be to have the user manually initiate the change of default rom once they are happy with the way it runs.

You can also get information about the boot from your application such as the boot mode (standard, temporary or GPIO), and the currently running rom. Previously the running rom could be determined by reading the config, but that would not work for a temporary boot.

This new functionality makes use of the ESP8266’s RTC data area and to use it uncomment the #define BOOT_RTC_ENABLED line in rboot.h. See the documentation in the GitHub rBoot repository and the updated sample project for example use.

One other change made at the same time is that GPIO-rom selection is now an optional feature and not compiled in by default. Please enable the appropriate #define in rboot.h if you wish to use this feature.

rBoot now in Sming

rBoot has now been integrated into Sming. This includes rBoot itself (allowing the bootloader to be built alongside a user app) and a new Sming specific rBoot OTA class. The sample app from my GitHub repo has been improved and included under the name Basic_rBoot, it demonstrates OTA updates, big flash support and multiple spiffs images.

The old sample app will be removed from my repo shortly. If you want to make use of rBoot in Sming it’s now easier than ever – just use release v1.3.0 onwards, or clone the Sming master branch, and take a look at the sample project.

C Version of system_rtc_mem_read

Here are C versions of system_rtc_mem_write & system_rtc_mem_write. You don’t need these when your app is compiled against the SDK but if you are running bare-metal code you might find them handy. You could add this code to rBoot so you can communicate with it between boots, e.g. setting a flag from your app to alter the behaviour of rBoot on next boot.

Note: in the SDK version if you specify a length that is not a multiple of 4 the actual length read will be rounded up and so it may overflow the supplied buffer (although alignment/packing of memory may mean this isn’t a problem), my version requires a read in multiples of 4 bytes, but you can easily remove that check if you wish.

uint32 system_rtc_mem_read(int32 addr, void *buff, int32 length) {

	int32 blocks;
	// validate reading a user block
	//if (addr < 64) return 0;
	if (buff == 0) return 0;
	// validate 4 byte aligned
	if (((uint32)buff & 0x3) != 0) return 0;
	// validate length is multiple of 4
	if ((length & 0x3) != 0) return 0;
	// check valid length from specified starting point
	if (length > (0x300 - (addr * 4))) return 0;

	// copy the data
	for (blocks = (length >> 2) - 1; blocks >= 0; blocks--) {
		volatile uint32 *ram = ((uint32*)buff) + blocks;
		volatile uint32 *rtc = ((uint32*)0x60001100) + addr + blocks;
		*ram = *rtc;

	return 1;

You’ll notice how similar these two functions are, if you need both you could easily combine them into a single function with a parameter to indicate read/write mode (which would save rom space).

uint32 system_rtc_mem_write(int32 addr, void *buff, int32 length) {

	int32 blocks;
	// validate reading a user block
	if (addr < 64) return 0;
	if (buff == 0) return 0;
	// validate 4 byte aligned
	if (((uint32)buff & 0x3) != 0) return 0;
	// validate length is multiple of 4
	if ((length & 0x3) != 0) return 0;
	// check valid length from specified starting point
	if (length > (0x300 - (addr * 4))) return 0;

	// copy the data
	for (blocks = (length >> 2) - 1; blocks >= 0; blocks--) {
		volatile uint32 *ram = ((uint32*)buff) + blocks;
		volatile uint32 *rtc = ((uint32*)0x60001100) + addr + blocks;
		*rtc = *ram;

	return 1;

C Version of system_get_rst_info

People keep asking me how to use SDK functions (e.g. system_get_rst_info) in rBoot so they can add interesting new features. The simplest answer is you can’t – if you want to use the full set of SDK features you would need to link rBoot against the SDK, it’s size would go from <4k to >200k and it wouldn’t actually be possible to chain load a user rom. The less simple answer is if the SDK functions are simple enough and can be reverse engineered then you can replicate them in rBoot, or any other bare-metal app. So some simple things will be possible, with work, but you’ll never be able to use more complex features like wifi from rBoot.

Here is C code to replicate the system_get_rst_info. Note that it passes back a structure rather than a pointer to one (the SDK creates and stores this structure at boot and later just passes a pointer to it when requested, but the code below creates it when needed). Also note that you cannot use this code after the SDK has started because the SDK resets this information when it boots, but it will work just fine in rBoot.

struct rst_info {
	uint32 reason;
	uint32 exccause;
	uint32 epc1;
	uint32 epc2;
	uint32 epc3;
	uint32 excvaddr;
	uint32 depc;

struct rst_info system_get_rst_info() {
	struct rst_info rst;
	system_rtc_mem_read(0, &rst, sizeof(struct rst_info);
	if (rst.reason >= 7) {
		ets_memset(&rst, 0, sizeof(struct rst_info));
	if (rtc_get_reset_reason() == 2) {
		ets_memset(&rst, 0, sizeof(struct rst_info));
		rst.reason = 6;
	return rst;

rBoot now supports Sming for ESP8266

Although it’s always been possible to use Sming compiled apps with rBoot it wasn’t easy. I’ve shared Makefiles and talked a few people through it previously on the forum, but now there is a new sample project on GitHub to help everyone do it.

The sample demonstrates:

  • Compiling a basic app (similar to the rBoot sample for the regular sdk).
  • Big flash support, allowing up to 4 roms each up to 1mb in size on an ESP12.
  • Over-the-air (OTA) updates.
  • Spiffs support, with a different filesystem per app rom.

Spiffs support depends on a patch to Sming, for which there is a pull request pending to have it included properly. Probably the most common way I envisage this being used is a pair of app roms (to allow for easy OTA updates ) with a separate spiffs file system each, but rBoot is flexible enough to let you lay out your flash however you want to.

Suggested layout for 4mb flash:

0x000000 rboot
0x001000 rboot config
0x002000 rom0
0x100000 spiffs0
0x1fc000 (4 unused sectors*)
0x200000 (2 unused sectors†)
0x202000 rom1
0x300000 spiffs1
0x3fc000 sdk config (last 4 sectors)

* The small unused section at the top of the second mb means the same size spiffs can be used for spiffs0 and spiffs1. The top of the fourth mb (where spiffs1 sits) is reserved for the sdk to store config.
† The small unused section at the start of the third mb mirrors the space used by rBoot at the start of the first mb. This means only one rom needs to be produced, that can be used in either slot, because it will be of the same size and have the same linker rom address.

C Version of Cache_Read_Enable for ESP8266

Just a quick post with a C version of the decompiled ESP8266 rom function Cache_Read_Enable. This function is responsible for memory mapping the SPI flash. I’ve previously discussed it, but a couple of people have wanted the code so it seemed worth posting here. This compiles to quite a few bytes which must stay in iram so, if you want to tamper with the parameters (like rBoot big flash support does), it’s best to write a wrapper to the original rom function rather than use this code to replace it.

void Cache_Read_Enable(uint8 odd_even, uint8 mb_count, uint8 no_idea) {
	uint32 base1 = 0x3FEFFE00;
	volatile uint32 *r20c = (uint32*)(base1 + 0x20c);
	volatile uint32 *r224 = (uint32*)(base1 + 0x224);
	uint32 base2 = 0x60000200;
	volatile uint32 *r008 = (uint32*)(base2 + 8);
	while (*r20c & 0x100) {
		*r20c &= 0xeff;

	*r008 &= 0xFFFDFFFF;
	*r20c &= 0x7e;
	*r20c |= 0x1;
	while ((*r20c & 0x2) == 0) {

	*r20c &= 0x7e;
	*r008 |= 0x20000;

	if (odd_even == 0) {
		*r20c &= 0xFCFFFFFF;  // clear bits 24 & 25
	} else if (odd_even == 1) {
		*r20c &= 0xFEFFFFFF;  // clear bit 24
		*r20c |= 0x2000000;   // set bit 25
	} else {
		*r20c &= 0xFDFFFFFF;  // clear bit 25
		*r20c |= 0x1000000;   // set bit 24

	*r20c &= 0xFBF8FFFF; // clear bits 16, 17, 18, 26
	*r20c |= ((no_idea << 0x1a) | (mb_count << 0x10)); // set bits 26 & 18/17/16
	// no_idea should be 0-1 (1 bit), mb_count 0-7 (3 bits)

	if (no_idea == 0) {
		*r224 |= 0x08; // set bit 3
	} else {
		*r224 |= 0x18; // set bits 3 & 4
	while ((*r20c & 0x100) == 0) {
		*r20c |= 0x100;