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Updated Comments about sleep() 

Clarification that after using sleep(), wake() must be the first function used to communicate with the flash chip.
If this isn't the case, a noisy MISO line can cause the code to hang in the while(busy) loop in the command() function.
This is a "fix" for "gitHub bug #26".
This commit is contained in:
JasonC0x0D 2021-06-07 20:07:28 -07:00 committed by GitHub
parent 6423a585c4
commit ac7bc9a993
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1 changed files with 16 additions and 6 deletions
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22
SPIFlash.cpp
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@ -38,8 +38,15 @@ uint8_t SPIFlash::UNIQUEID[8];
/// IMPORTANT: NAND FLASH memory requires erase before write, because
/// it can only transition from 1s to 0s and only the erase command can reset all 0s to 1s
/// See http://en.wikipedia.org/wiki/Flash_memory
/// The smallest range that can be erased is a sector (4K, 32K, 64K); there is also a chip erase command
/// See http://en.wikipedia.org/wiki/Flash_memory
/// The smallest range that can be erased is a sector (4K, 32K, 64K); there is also a chip erase command
/// IMPORTANT: When flash chip is powered down, aka sleeping, the only command it will respond to is
/// Release Power-down / Device ID (ABh), per section 8.2.19 of the W25X40CL datasheet.
/// This means after using the sleep() function of this library, wake() must be the first
/// function called. If other commands are used, the flash chip will ignore the commands.
/// The risk is that the code can hang if there is noise/static on the MISO line,
/// specifically noise/static that would evaluate to HIGH or 1.
/// Constructor. JedecID is optional but recommended, since this will ensure that the device is present and has a valid response
/// get this from the datasheet of your flash chip
@ -180,11 +187,14 @@ void SPIFlash::command(uint8_t cmd, boolean isWrite){
command(SPIFLASH_WRITEENABLE); // Write Enable
unselect();
}
//wait for any write/erase to complete
// wait for any write/erase to complete
// a time limit cannot really be added here without it being a very large safe limit
// that is because some chips can take several seconds to carry out a chip erase or other similar multi block or entire-chip operations
// a recommended alternative to such situations where chip can be or not be present is to add a 10k or similar weak pulldown on the
// open drain MISO input which can read noise/static and hence return a non 0 status byte, causing the while() to hang when a flash chip is not present
//
// Note: If the MISO line is high, busy() will return true.
// This can be a problem and cause the code to hang when there is noise/static on MISO data line when:
// 1) There is no flash chip connected
// 2) The flash chip connected is powered down, aka sleeping.
if (cmd != SPIFLASH_WAKE) while(busy());
select();
SPI.transfer(cmd);
@ -305,4 +315,4 @@ void SPIFlash::wakeup() {
/// cleanup
void SPIFlash::end() {
SPI.end();
}
}