AT91FR40161 ATMEL Corporation, AT91FR40161 Datasheet - Page 27

• Manufacturer Part Number: AT91FR40161
• Description: The AT91FR40161 Features 136 K Bytes of On-chip SRAM, 2M Bytes of Flash, an External Bus Interface, a 3-channel Timer/Counter, 2 ...
• Manufacturer: ATMEL Corporation

Read Protocols
Standard Read Protocol
Early Read Protocol
Early Read Wait State
1354D–ATARM–08/02
The EBI provides two alternative protocols for external memory read access: standard
and early read. The difference between the two protocols lies in the timing of the NRD
(read cycle) waveform.
The protocol is selected by the DRP field in EBI_MCR (Memory Control Register) and is
valid for all memory devices. Standard read protocol is the default protocol after reset.
Note:
Standard read protocol implements a read cycle in which NRD and NWE are similar.
Both are active during the second half of the clock cycle. The first half of the clock cycle
allows time to ensure completion of the previous access as well as the output of address
and NCS before the read cycle begins.
During a standard read protocol, external memory access, NCS is set low and ADDR is
valid at the beginning of the access while NRD goes low only in the second half of the
master clock cycle to avoid bus conflict (see Figure 14). NWE is the same in both proto-
cols. NWE always goes low in the second half of the master clock cycle (see Figure 15).
Early read protocol provides more time for a read access from the memory by asserting
NRD at the beginning of the clock cycle. In the case of successive read cycles in the
same memory, NRD remains active continuously. Since a read cycle normally limits the
speed of operation of the external memory system, early read protocol can allow a
faster clock frequency to be used. However, an extra wait state is required in some
cases to avoid contentions on the external bus.
In early read protocol, an early read wait state is automatically inserted when an exter-
nal write cycle is followed by a read cycle to allow time for the write cycle to end before
the subsequent read cycle begins (see Figure 16). This wait state is generated in addi-
tion to any other programmed wait states (i.e. data float wait).
No wait state is added when a read cycle is followed by a write cycle, between consecu-
tive accesses of the same type or between external and internal memory accesses.
Early read wait states affect the external bus only. They do not affect internal bus timing.
Figure 14. Standard Read Protocol
In the following waveforms and descriptions, NRD represents NRD and NOE since the
two signals have the same waveform. Likewise, NWE represents NWE, NWR0 and
NWR1 unless NWR0 and NWR1 are otherwise represented. ADDR represents A0 - A23
and/or A1 - A23.
or
ADDR
MCKI
NWE
NCS
NRD
AT91X40 Series
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