ADSP-2126x SHARC Processor Hardware Reference
8-11
Parallel Port
Comparison of 16-Bit and 8-Bit SRAM Modes
When considering whether to employ the 16- or 8-bit mode in a particu-
lar design, a few key points should be considered.
• The 8-bit mode provides a 24-bit address, and therefore can access
16M bytes of external memory. In contrast, the 16-bit mode can
only address 64K x 16 bit words, which is equivalent to 128K
bytes. Therefore, the 8-bit mode provides 128 times the storage
capacity of the 16-bit mode.
• For sequential accesses, the 8-bit mode requires only one
ALE
cycle
per 256 bytes. With minimum wait states selected, this represents a
worst case overhead of:
(1
ALE
cycle)/(256 ac 1
ALE
) x 100% = 0.39% overhead for
ALE
cycles. In contrast, the 16-bit mode requires one
ALE
cycle per
external sequential access. Regardless of length (N), this represents
a worst case overhead of:
(N
ALE
cycles)/(N ac N
ALE
cycles) x 100% = 50% overhead
for
ALE
cycles. However, the 16-bit mode delivers two bytes per
cycle. Therefore, the total data transfer speed for sequential
accesses is nearly identical for both 8-bit and 16-bit modes.
The question that arises at this point is: If the total transfer rates are the
same for both 8-bit and 16-bit modes, and the 8-bit mode can also address
128 times as much external memory, why would a system use the 16-bit
mode?
• Sometimes an external device is only capable of interfacing to a
16-bit bus.
• When the DMA external modifier is set to zero, the address does
not change after the first cycle, therefore an
ALE
cycle is only
inserted on the first cycle. In this case, the 16-bit port can run
Summary of Contents for ADSP-21261 SHARC
Page 30: ...Contents xxx ADSP 2126x SHARC Processor Hardware Reference ...
Page 40: ...Register Diagram Conventions xl ADSP 2126x SHARC Processor Hardware Reference ...
Page 58: ...Differences From Previous SHARCs 1 18 ADSP 2126x SHARC Processor Hardware Reference ...
Page 112: ...Secondary Processing Element PEy 2 54 ADSP 2126x SHARC Processor Hardware Reference ...
Page 178: ...Summary 3 66 ADSP 2126x SHARC Processor Hardware Reference ...
Page 204: ...DAG Instruction Summary 4 26 ADSP 2126x SHARC Processor Hardware Reference ...
Page 322: ...Setting Up DMA 7 32 ADSP 2126x SHARC Processor Hardware Reference ...
Page 436: ...SPORT Programming Examples 9 86 ADSP 2126x SHARC Processor Hardware Reference ...
Page 521: ...ADSP 2126x SHARC Processor Hardware Reference 11 31 Input Data Port rts IDP_ISR end ...
Page 522: ...Input Data Port Programming Example 11 32 ADSP 2126x SHARC Processor Hardware Reference ...
Page 590: ...Timer Programming Examples 14 20 ADSP 2126x SHARC Processor Hardware Reference ...
Page 796: ...I O Processor Registers A 174 ADSP 2126x SHARC Processor Hardware Reference ...
Page 800: ...B 4 ADSP 2126x SHARC Processor Core Manual ...
Page 846: ...Index I 36 ADSP 2126x SHARC Processor Hardware Reference ...