Atmel Evaluation Kit AT91SAM9X25-EK AT91SAM9X25-EK Data Sheet

393

SAM9X25 [DATASHEET]

11054E–ATARM–10-Mar-2014

As NCS and NRD waveforms are defined independently of one other, the SMC needs to know when the read data is 
available on the data bus. The SMC does not compare NCS and NRD timings to know which signal rises first. The 
READ_MODE parameter in the SMC_MODE register of the corresponding chip select indicates which signal of NRD and 
NCS controls the read operation.

 shows the waveforms of a read operation of a typical asynchronous RAM. The read data is available t

PACC

 

after the falling edge of NRD, and turns to ‘Z’ after the rising edge of NRD. In this case, the READ_MODE must be set to 
1 (read is controlled by NRD), to indicate that data is available with the rising edge of NRD. The SMC samples the read 
data internally on the rising edge of Master Clock that generates the rising edge of NRD, whatever the programmed 
waveform of NCS may be.

PACC 

after the falling edge of the 

NCS signal and remains valid until the rising edge of NCS. Data must be sampled when NCS is raised. In that case, the 
READ_MODE must be set to 0 (read is controlled by NCS): the SMC internally samples the data on the rising edge of 
Master Clock that generates the rising edge of NCS, whatever the programmed waveform of NRD may be.

Data Sampling

t

PACC

MCK

A[25:2]

NBS0,NBS1,
NBS2,NBS3,
A0, A1

NCS

NRD

D[31:0]