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All - Their are both historical and practical reasons for the default values in the SDK in regards to the PLL multiplier associated with each component that might help. Originally the PLL mulitplier and the DSP56F80x components was set at 72 Mhz to best meet what we believed to be the fastest rate at which the combination of worst case timing in conjunction with the speed of the RAM on the EVMs and for another reason that really had nothing to do with the rate at which external RAM could run. This was thought to be the safest approach. Latter we began to modify our thinking some. We also modify the PLL mulitplier to access external memory at full rate and do not experience problems. This works because of what Art has described that when you are not at worst case voltage and temperature the performance the components are much better than in the data sheet. To me this is a perfectly valid thing to do while in development. Run at full rate and use the extra memory and speed available to debug your application. Once you are go to your final target throttle down the speed of any external memory accesses to the rate that satisfies the speed of ! th! e external RAM you have chosen. In the case of the DSP56F826/827 EVM we set the PLL multiplier to run at full rate even though this doesn't meet the worst case data sheet specifications of the RAM on the 826/827 EVM. But emperatical testing have shown no problems and the EVM is intended only for development. We no longer felt that 72 Mhz was the proper default because this was probably only unneccessarily running customers programs at a slower rate than was appropriate for their final target hardware and confusing customers evalauting the parts as to the best possibel performance. We set the default rates on the 826/827 EVMs to be full rate because of these reasons. In regards to the DSP56F80x did not modify the deafult rate because the product had already been fielded and we try quite hard to not make modifiactions in the SDK that could have unexpected results to existing customers. The bottom line is that you need to set the PLL multiplier to be correct for your end system and its end environment. But while in development in can be beneficial to break the data sheet rules and in this case it works out when accessing external RAM faster. Also in regards to the DSP56F826/827 we have recently posted a FAQ on accessing slower external RAM at a faster rate than specified in the data sheet by limiting the capacitance of the external memory lines to 20 pf instead of the data sheet test conditions of 50 pf. Thanks. - Bill -----Original Message----- From: Art Johnson [mailto:] Sent: Friday, December 06, 2002 12:59 PM To: Johnson, Jerry; Roger Flor; Subject: RE: [motoroladsp] SDK default CPU clock rate, DSP56F80x The 80MHz clock rate will probably work in most if not all systems, due to the fact that most RAM chips have access times that are better than the "worst-case" value that is specified on the data sheet. This does not, however, guarantee that it will work for all devices under all combinations of power supply voltage and temperature. The only way to guarantee that, is to compare the specified maximum access times and minimum pulse width times of the memory device (including any delays due to address decoding and/or address/data bus buffers), to the required "zero-wait-state" times in the DSP56F805 data sheet. Of course, on the EVM module, there are none of these "extra" delays, so the calculation is as follows: 1) From going through the DSP56F805/D Rev. 8.0, 11/2002 data sheet, and the GS72116TP Rev. 1.04a 10/2002 data sheet, the most critical timing parameter is the minimum write pulse width, as shown below: DSP56F807 80MHz minimum write pulse width = 7.5ns GS72116TP-12 minimum write pulse width = 8.0ns GS72116TP-10 minimum write pulse width = 7.0ns GS72116TP-8 minimum write pulse width = 5.5ns GS72116TP-7 minimum write pulse width = 5.0ns 2) Since the DSP56F805EVM module uses the 12-nanosecond GS72116TP-12 RAM chip, it is not guaranteed to work with zero wait states at a processor clock frequency of 80MHz. The 10-nanosecond GS72116TP-10 RAM chip would be required in this case. Most of the experienced hardware design engineers I am acquainted with would require at least a 1-nanosecond margin to be on the safe side, so this would require you to use the 8-nanosecond GS72116TP-8 RAM chip. 3) While your chance of running across a case where the 12-nanosecond chip actually caused a failure is quite low, it is not zero. If your systems are used in critical control, monitoring, and safety applications as ours are, then even this very small possibility of failure is unacceptable. Especially when the potential for failure is easily demonstrated by a simple calculation like the one shown above. Regards, Art Johnson Senior Systems Analyst PMC Prime Mover Controls Inc. 3600 Gilmore Way Burnaby, B.C., Canada V5G 4R8 Phone: 604 433-4644 FAX: 604 433-5570 Email: http://www.pmc-controls.com <http://www.pmc-controls.com -----Original Message----- From: Johnson, Jerry [mailto:] Sent: Friday, December 06, 2002 6:20 AM To: 'Roger Flor'; Subject: RE: [motoroladsp] SDK default CPU clock rate, DSP56F80x There have been earlier postings about the speed issue, and the default. As far as I know there are no problems with 80Mhz operation. I have been initializing it that way for over a 18 months, on several hardware platforms. At the higher internal clock rate, there is more power dissapation, but I have not seen any issues yet. The peripheral bus rate will default to 40Mhz instead of 36 Mhz, but that is probably what you want to happen. -----Original Message----- From: Roger Flor [mailto:] Sent: Thursday, December 05, 2002 6:07 PM To: Subject: [motoroladsp] SDK default CPU clock rate, DSP56F80x Hello! I am using a DSP56F805 EVM. Page 3-47 of the Embedded SDK Targeting Motorola DSP56F80x Platform manual says that the default SDK setting for the CPU clock is 72MHz. Is there any special reason for this? Why not 80MHz? Is there anything I should be aware of if I set the CPU clock to 80 MHz? Thanks in advance Roger _________________________________________________________________ The new MSN 8: advanced junk mail protection and 2 months FREE* http://join.msn.com/?pageatures/junkmail <http://join.msn.com/?pageatures/junkmail _____________________________________ Note: If you do a simple "reply" with your email client, only the author of this message will receive your answer. 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