Information

Assuming zero wait states on the system bus, DMA requests can be processed every 9
cycles. Assuming an average of the access times associated with internal peripheral bus-
to-SRAM (4 cycles) and SRAM-to-internal peripheral bus (5 cycles), DMA requests can
be processed every 11.5 cycles (4 + (4+5)/2 + 3). This is the time from Cycle 4 to Cycle ?
+5. The resulting peak request rate, as a function of the system frequency, is shown in the
following table.
Table 21-295. eDMA peak request rate (MReq/sec)
System frequecy (MHz)
Request rate
with zero wait states
Request rate
with wait states
66.6 7.4 5.8
83.3 9.2 7.2
100.0 11.1 8.7
133.3 14.8 11.6
150.0 16.6 13.0
A general formula to compute the peak request rate with overlapping requests is:
PEAKreq = freq / [ entry + (1 + read_ws) + (1 + write_ws) + exit ]
where:
Table 21-296. Peak request formula legend
Where Represents
PEAKreq Peak request rate
freq System frequency
entry Channel startup (4 cycles)
read_ws Wait states seen during the system bus read data phase
write_ws Wait states seen during the system bus write data phase
xit Channel shutdown (3 cycles)
For example, consider a system with the following characteristics:
• Internal SRAM can be accessed with one wait-state when viewed from the system
bus data phase
• All internal peripheral bus reads require two wait-states, and internal peripheral bus
writes three wait-states viewed from the system bus data phase
• System operates at 150 MHz
For an SRAM to internal peripheral bus transfer,
Functional description
K20 Sub-Family Reference Manual, Rev. 6, Nov 2011
478 Freescale Semiconductor, Inc.