Information

be processed every 11.5 cycles (4 + (4+5)/2 + 3). This is the time from Cycle 4 to Cycle x
+5. The resulting peak request rate, as a function of the system frequency, is shown in the
following table.
Table 21-103. eDMA peak request rate (MReq/sec)
System frequency (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 21-104. Peak request formula operands
Operand Description
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
exit Channel shutdown (3 cycles)
21.4.4.3 eDMA performance 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. 2, Feb 2012
380 Freescale Semiconductor, Inc.