Atmel Xplained Pro Evaluation Kit ATSAM4E-XPRO ATSAM4E-XPRO Data Sheet

SAM4E [DATASHEET]

Atmel-11157D-ATARM-SAM4E16-SAM4E8-Datasheet_12-Jun-14

544

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 (t

DF

) for each external memory device is programmed in the TDF_CYCLES field of the

SMC_MODE register for the corresponding chip select. The value of 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 t

DF 

will not slow down the execution of a program from internal memory. 

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

Setting the READ_MODE to 1 indicates to the SMC that the NRD signal is responsible for turning off the tri-state
buffers of the external memory device. The Data Float Period then begins after the rising edge of the NRD signal
and lasts TDF_CYCLES MCK cycles. 

When the read operation is controlled by the NCS signal (READ_MODE = 0), the TDF field gives the number of
MCK cycles during which the data bus remains busy after the rising edge of NCS. 

 illustrates the Data Float Period in NRD-controlled mode (READ_MODE =1), assuming a data float

period of 2 cycles (TDF_CYCLES = 2). 

 shows the read operation when controlled by NCS

(READ_MODE = 0) and the TDF_CYCLES parameter equals 3. 

NCS

NRD controlled read operation

tpacc

MCK

NRD

D[7:0]

TDF = 2 clock cycles

A[23:0]