6-48 Intel® PXA255 Processor Developer’s Manual

Memory Controller

Table 6 -22 provides a c omparison of supporte d Asynchronous Static Memo ry types.

6.7.3 ROM Interface

The processor provides programmable timing for both burst and non-burst ROMs. The RDF field

in MSCx is the latency (in memory clock cycles) for the first, and all subsequent, data beats from

non-burst ROMs, and the first data beat from a burst ROM. RDN is the latency for the burst data

beats after the first for burst ROMs. RRR delays the following access to a different memory space

to allow time for the current ROM to three-state the data bus.

RRR must be programmed with the maximum tOFF value, as specified by the ROM manufacturer.

For hardware reset initialization values, refer to Section6.8. MSC0[15:0] is selected when the

address space corresponding to nCS0 is accessed. The processor supports a ROM burst si ze of 1, 4,

or 8 by configuring the MSCx[RTx] registe r bits to 0, 2 or 3 respectfully.

Table 6-22. Asynchronous Static Memory and Variable Latency I/O Capabilities

MSCx[RTx] Device
Type
Timing (Memory Clocks)
Burst
Read
Address
Assert
nOE
Assert
Burst
nOE
Deassert
Burst
Write
Address
Assert
nWE
Assert
Burst
nWE
De-
assert
000
Non-burst
ROM or
Flash
RDF+1 RDF+1 0 N/A R DF+1 N/A
001 SRAM RDF+1 RDF+1 0 RDN+2 RDN+1 1
010
Burst-of-4
ROM or
Flash (non-
burst writes)
RDF+1
(0,4)
RDN+1
(1:3,5:7)
RDF+1
(0,4)
RDN+1
(1:3,5:7)
0 N/A RDF+1 N/A
011
Burst-of-8
ROM or
Flash
(non-burst
writes)
RDF+1
(0)
RDN+1
(1:7)
RDF+1
(0)
RDN+1
(1:7)
0 N/A RDF+1 N/A
100 Varia ble
Latency I/O
RDF+
RDN+2+waits
RDF+1+
waits RDN+2 RDF+
RDN+2+waits
RDF+1+
waits RDN+2