Atmel Evaluation Kit AT91SAM9G25-EK AT91SAM9G25-EK データシート

383

SAM9G25 [DATASHEET]

11032C–ATARM–25-Jan-13

Due to an internal mechanism, a wait cycle is always inserted between consecutive read and write SMC accesses. 

This wait cycle is referred to as a read to write wait state in this document.

This wait cycle is applied in addition to chip select and reload user configuration wait states when they are to be inserted.
See 

.

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:

z

before starting a read access to a different external memory

z

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.