AN2505 Freescale Semiconductor / Motorola, AN2505 Datasheet - Page 12

• Manufacturer Part Number: AN2505
• Description: MSC8102 Asynchronous DSI Throughput
• Manufacturer: Freescale Semiconductor / Motorola

Actual Throughput Versus Frequency
5.2 Single Write Versus Local Bus Frequency
Figure 12 illustrates the DSI throughput for a single write transaction versus the local bus frequency of the
MSC8102. The DSI operates on the basis of the local bus frequency. The maximum observed throughput occurs
when the local bus frequency is 70 MHz or higher. The throughput versus frequency for a single write is much
different than for a single read. The single write throughput stays constant until the frequency drops below 30
MHz. At 20 MHz the throughput drops about 15 MB/s from the maximum throughput, which is a 33 percent
decrease. The single write at 20 MHz is clearly lower than the single read at 20 MHz. This is in accordance with
the 70 MHz data in which the single write throughput is less than the single read throughput. The single read data
shows a 30 percent decrease over the entire frequency range, whereas the single write data falls off sharply at 30
MHz and shows a 33 percent decrease.
5.3 Host DMA Read Versus Local Bus Frequency
Figure 13 illustrates the DSI throughput for a host DMA read transaction versus the local bus frequency of the
MSC8102. This chart illustrates that the observed maximum throughput occurs when the local bus frequency is 70
MHz or higher. Notice that at 20 MHz, the host DMA read is only 3 MB/s more than for a single read transaction.
The reduction in MSC8102 local bus frequency affects the advantages gained by the MSC8101 DMA controller
over the single read transactions. However, the DMA controller is still preferable since it does not stall the
MSC8101 core, as a normal move instruction does.
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Figure 12. DSI Single Write Throughput Versus MSC8102 Local Bus Frequency


MSC8102 Asynchronous DSI Throughput, Rev. 1

8102 Local Bus Frequency (MHz)




Freescale Semiconductor