Datasheet
35.12 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 (t
DF
) 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 t
DF
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.
35.12.1 SMC_MODE.READ_MODE
Setting SMC_MODE.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 SMC_MODE.TDF_CYCLES MCK cycles.
When the read operation is controlled by the NCS signal (SMC_MODE.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.
Figure 35-21 illustrates the Data Float Period in NRD-controlled mode (SMC_MODE.READ_MODE =1), assuming a
data float period of 2 cycles (SMC_MODE.TDF_CYCLES = 2). Figure 35-22 shows the read operation when
controlled by NCS (SMC_MODE.READ_MODE = 0) and SMC_MODE.TDF_CYCLES = 3.
Figure 35-21. TDF Period in NRD Controlled Read Access (TDF = 2)
NCS
NRD controlled read operation
tpacc
MCK
NRD
D[7:0]
TDF = 2 clock cycles
A[23:0]
SAM E70/S70/V70/V71 Family
Static Memory Controller (SMC)
©
2019 Microchip Technology Inc.
Datasheet
DS60001527D-page 432