34.12 Data Float Wait States

Some memory devices are slow to release the external bus. For such devices, it is necessary to add wait states (data float wait states) after a read access:

  • before starting a read access to a different external memory
  • before starting a write access to the same device or to a different external one.

The data float output time (tDF) for each external memory device is programmed in the SMC_MODE.TDF_CYCLES field for the corresponding chip select. The value of SMC_MODE.TDF_CYCLES indicates the number of data float wait cycles (between 0 and 15) before the external device releases the bus, and represents the time allowed for the data output to go to high impedance after the memory is disabled.

Data float wait states do not delay internal memory accesses. Hence, a single access to an external memory with long tDF will not slow down the execution of a program from internal memory.

The data float wait states management depends on SMC_MODE.READ_MODE and the SMC_MODE.TDF_MODE fields for the corresponding chip select.