Sorry for the length of this question, but I thought it best to show as much detail to fend of questions asking if I had done A when I had already done A... ;-) I've had a look at the "micro-SD card initialization using SPI interface" thread and didn't see any answers that matched my issue (i.e. things I haven't already tried). I have a similar issue where I'm trying to access a SD card through a micro-controller's SPI interface (specifically an HC908). I've tried following the flow charts in the Physical Layer Simplified Specification v2.00 and it seems to initialize correctly on Transcend 1GB & 2GB and an AE&C 1GB card. But I'm having problems on 3 other random cards from my stash of old cards that I've used on my camera. My code is all HC908 assembler. I scoped out the SPI clock line and during initialization it's running about 350kHz (the only speed multiplier that the HC908 supplies at my low MCU clock speed that falls within the 100-400kHz window). Here are the results of the three cards that aren't completing my initialization routine (all done consecutively w/o changing any code or timing parameters): Canon 16Meg card (labeled as SD): Set card select high Send 80 SPI clock cycles (done by writing 0xFF 10 times) Set card select low Send CMD0 [0x400000000095] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x01 (indicates idle) Send CMD8 [0x48000001AA87] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x05 (idle and illegal command) Because illegal command set local flag to indicate v1 or MMC card Send CMD58 [0x7A00000000FD] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x05 (idle and illegal command) because illegal command branch to error routine Send CMD13 [0x4D000000000D] (show status buffer) and Loop up to 8 times waiting for high bit on response to go low R1= 0x05 (idle and illegal command) Is the illegal command flag stuck? Should I be doing something after CMD8 to clear that flag? SanDisk UltraII 256Meg Set card select high Send 80 SPI clock cycles (done by writing 0xFF 10 times) Set card select low Send CMD0 [0x400000000095] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x01 (idle) Send CMD8 [0x48000001AA87] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x05 (idle and illegal command) Because illegal command set local flag to indicate v1 or MMC card Send CMD58 [0x7A00000000FD] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x01 (idle) Send 0xFF 4 times to read OCR OCR = 0xFFFFFFFF Send CMD55 [0x770000000065] (1st part of ACMD41) and Loop up to 8 times waiting for high bit on response to go low R1 = 0x01 (idle) Send CMD41 [0x6900000000E5] (2nd part of ACMD41) and Loop up to 8 times waiting for high bit on response to go low R1 = 0x05 (idle and illegal command) Because illegal command, assume card is MMC Send CMD1 [0x4100000000F9] (for MMC) and Loop up to 8 times waiting for high bit on response to go low R1 = 0x05 (idle and illegal command) Repeat the CMD1 50 times (my arbitrary number to wait until idle clears) Every R1 response is 0x05 (idle and illegal command) Why is OCR all F? Doesn't seem proper at all. Also, why does ACMD41 and CMD1 respond illegal command? Is CMD1 failing because the card is waiting for a valid ACMD after the CMD55 even with the illegal command response? SanDisk ExtremeIII 2G: Set card select high Send 80 SPI clock cycles (done by writing 0xFF 10 times) Set card select low Send CMD0 [0x400000000095] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x01 (idle) Send CMD8 [0x40000001AA87] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x7F (??? My loop shows the responses for each iteration and I got 0xFF 0xFF 0xC1 0x7F... is the card getting out of sync?) Send CMD58 [0x7A00000000FD] and Loop up to 8 times waiting for high bit on response to go low R1 = 0x01 (idle and back in sync) Send 0xFF 4 times to read OCR OCR = 0x00FF80 Send CMD55 [0x770000000065] (1st part of ACMD41) and Loop up to 8 times waiting for high bit on response to go low R1 = 0x5F (??? loop responses are 0xFF 0xFF 0xF0 0x5F... again out of sync?) Send CMD41 [0x6900000000E5] (2nd part of ACMD41) and Loop up to 8 times waiting for high bit on response to go low R1 = 0x05 (idle and illegal command, but back in sync???) Because illegal command, assume card is MMC Send CMD1 [0x4100000000F9] (for MMC) and Loop up to 8 times waiting for high bit on response to go low R1 = 0x7F (??? loop responses are 0xFF 0xFF 0xC1 0x7F... again out of sync?) Repeat CMD1 and Loop up to 8 times waiting for high bit on response to go low R1 = 0x01 (idle) Repeat CMD1 and Loop up to 8 times waiting for high bit on response to go low R1 = 0x7F (??? loop responses are 0xFF 0xFF 0xC1 0x7F... again out of sync?) Repeat CMD1 and Loop up to 8 times waiting for high bit on response to go low R1 = 0x00 (out of idle) Send CMD9 [0x4900000000AF] (get CSD) and Loop up to 8 times waiting for high bit on response to go low R1 = 0x3F (??? loop responses are 0xFF 0xFF 0xC1 0x3F... again out of sync?) Code craps out because Illegal command bit is high. WTF is wrong with that card? Sometimes in sync, other times not. (The above pattern is repeatable.) I've scoped this one out and I'm not seeing any rogue clock cycles going through between MOSI/MISO transfers. Anyone have any clues? Need any more info? Thanx in advance for spending the time to read through all of this.