SAM3X8E Atmel Corporation, SAM3X8E Datasheet - Page 440

• Manufacturer Part Number: SAM3X8E
• Manufacturer: Atmel Corporation

26.10.1.2
26.10.1.3
26.10.2
26.10.2.1
440
440
SAM3X/A
SAM3X/A
Read Mode
NCS Waveform
Read Cycle
Read is Controlled by NRD (READ_MODE = 1):
Similarly, the NCS signal can be divided into a setup time, pulse length and hold time:
The NRD_CYCLE time is defined as the total duration of the read cycle, i.e., from the time where
address is set on the address bus to the point where address may change. The total read cycle
time is equal to:
NRD_CYCLE = NRD_SETUP + NRD_PULSE + NRD_HOLD
= NCS_RD_SETUP + NCS_RD_PULSE + NCS_RD_HOLD
All NRD and NCS timings are defined separately for each chip select as an integer number of
Master Clock cycles. To ensure that the NRD and NCS timings are coherent, the user must
define the total read cycle instead of the hold timing. NRD_CYCLE implicitly defines the NRD
hold time and NCS hold time as:
NRD_HOLD = NRD_CYCLE - NRD SETUP - NRD PULSE
NCS_RD_HOLD = NRD_CYCLE - NCS_RD_SETUP - NCS_RD_PULSE
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 tim-
ings 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.
Figure 26-8
data is available t
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 wave-
form of NCS may be.
1. NCS_RD_SETUP: the NCS setup time is defined as the setup time of address before
2. NCS_RD_PULSE: the NCS pulse length is the time between NCS falling edge and
3. NCS_RD_HOLD: the NCS hold time is defined as the hold time of address after the
the NCS falling edge.
NCS rising edge.
NCS rising edge.
shows the waveforms of a read operation of a typical asynchronous RAM. The read
PACC
after the falling edge of NRD, and turns to ‘Z’ after the rising edge of NRD.
11057A–ATARM–17-Feb-12
11057A–ATARM–17-Feb-12