Chip Select Operation

Table 12-3. Accesses by Matches in CSARs and DACRs

Number of CSCR Matches

Number of DACR Matches

Type of Access

 

 

 

0

0

External

 

 

 

1

0

Defined by CSAR

 

 

 

Multiple

0

External, burst-inhibited, 32-bit

 

 

 

0

1

Defined by DACRs

 

 

 

1

1

Undefined

 

 

 

Multiple

1

Undefined

 

 

 

0

Multiple

Undefined

 

 

 

1

Multiple

Undefined

 

 

 

Multiple

Multiple

Undefined

 

 

 

12.3.1.1 8-, 16-, and 32-Bit Port Sizing

Static bus sizing is programmable through the port size bits, CSCR[PS]. See Section 12.4.1.3 for more information. Figure 12-1shows the correspondence between the data bus and the external byte strobe control lines (BS[3:0]). Note that all byte lanes are driven, although the state of unused byte lanes is undefined.

External data bus

32-bit port memory

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

BS3

 

 

BS2

 

BS1

 

BS0

 

 

 

 

 

 

 

 

 

 

 

 

D[31:24]

D[23:16]

D[15:8]

D[7:0]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Byte 0

Byte 1

Byte 2

Byte 3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

16-bit port memory

8-bit port memory

Byte 0

Byte 1

Driven, undefined

Byte 2

Byte 3

 

Byte 0

 

Byte 1

Driven, undefined

Byte 2

 

Byte 3

 

Figure 12-1. Connections for External Memory Port Sizes

12.3.1.2 External Boot Chip Select Operation

CS0, the external boot chip select, allows address decoding for boot ROM before system initialization. Its operation differs from other external chip select outputs after system reset.

After system reset, CS0 is asserted for every external access. No other chip select can be used until the valid bit, CSMR0[V], is set, at which point CS0 functions as configured and

12-4

MCF5282 User’s Manual

MOTOROLA

Page 258
Image 258
Motorola MCF5282, MCF5281 12.3.1.1 8-, 16-, and 32-Bit Port Sizing, External Boot Chip Select Operation, BS2 BS1 BS0