SPRZ436G October 2015 – February 2021 AM5706 , AM5708 , AM5716 , AM5718 , AM5718-HIREL
QSPI Reads Can Fail For Flash Devices with HOLD Function
High
The default internal pull-up/pull-down on the SoC QSPI interface can interfere with the HOLD function implemented in some QSPI FLASH devices leading to Read Failures. This is most likely to be seen at higher clock rates, and with EDMA reads of greater than 128-Bytes.
In Quad SPI mode, the SoC QSPI IP transmits the command and address to the flash device on data line D0 and reads the data back on all four data lines D0, D1, D2 and D3. The default values of the data lines i.e. values when there is no driver on lines are LOW for D0 and D1 and HIGH for D2 and D3. These values are dictated by the internal and external pull ups.
When the last bit on the last read driven by the FLASH doesn't match the 'default value', the data lines D1, D2 and D3 transition slowly to their default values i.e. LOW for D1 and HIGH for D2 and D3. The transition time is in the order of 100 ns and depends on board loadings. At higher frequencies (typically above 64MHz QSPI clock rate) the time from the last bit of data transfer to the first bit of the next command is not long enough to allow for the pull-ups to get the data lines D1, D2 and D3 to the desired state. It is possible that the D3 line is still in a LOW state when the next command transmission begins.
The D3 line is used by some flash devices as a HOLD signal. If the D3 line has not reached HIGH state by the time CS is reasserted, flash devices can infer that a HOLD is in effect and fail to service the current command.
This issue is most easily seen with EDMA reads of length greater than 128 bytes. CPU reads typically provide sufficient time between reads for the data lines to reach their default values.
The software workaround is to disable the hold functionality on the QSPI device, preferably by setting a nonvolatile configuration register. On most flash devices, placing the QSPI device in Quad read mode automatically disables the HOLD functionality. On certain flash devices, there is a separate mode bit that can be set to disable the HOLD functionality. Typically this setting would be done in a Flash Writer utility that programs the flash with the customer’s boot image and sets appropriate non-volatile mode bits. Depending on the software architecture, this mode bit setting may also be done in the boot-loader or HLOS kernel.
Disabling the HOLD functionality prevents the slow ramp on the D3 line from interrupting the operation of the QSPI flash device and allows EDMA reads at high clock speeds (64 MHz).
There are no negative effects of the workaround as HOLD functionality is not supported by the SoC QSPI IP.
AM571x SR 2.1, 2.0, 1.0
AM570x SR 2.1, 2.0
DRA79x: 2.1, 2.0
TDA2Ex (23mm): 2.0, 1.0
TDA2Ex (17mm): 2.1, 2.0
AM571x: 2.1, 2.0, 1.0
AM570x: 2.1, 2.0
DRA72x: 2.0, 1.0
DRA71x: 2.1, 2.0