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ARM9 replacing Cortex M3

Michael von Hauff
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  • 0 Arm Employee Badge
    Note: This was originally posted on 8th January 2009 at http://forums.arm.com

    Bear in mind that on ARM, a word is 32 bits. Therefore, 18Mwords/s equates to 72MB/s.

    If your DACs are external, your biggest problem is likely to be the rate at which you can drive the physical pins on the chip. The GPIO peripherals on most MCUs are fairly slow as it takes a relatively large amount of power to drive them quickly. Of course, this depends on how you are interfacing with the DAC, but most serial protocols implemented in MCU peripherals will not be able to transfer data that fast. Typically, they require at least two bus clocks for each bit that is sent, and they tend to have additional overheads too.

    If the DACs are on-chip, you may not have this problem. On-chip DACs often don't have a sufficiently high precision for some applications, but you said that you only require 8 bits so if they're fast enough they may be worth considering. Also bear in mind that many on-chip DACs can be made to run faster at the cost of some precision, so a 10-bit DAC that isn't quite fast enough may be configurable to allow it to run faster if you accept that the lowest two bits will not be correct. This is certainly the case with the ADCs on NXP's LPC devices as I've used them before, and DACs are often similar in implementation to ADCs.
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  • 0 Arm Employee Badge
    Note: This was originally posted on 8th January 2009 at http://forums.arm.com

    Bear in mind that on ARM, a word is 32 bits. Therefore, 18Mwords/s equates to 72MB/s.

    If your DACs are external, your biggest problem is likely to be the rate at which you can drive the physical pins on the chip. The GPIO peripherals on most MCUs are fairly slow as it takes a relatively large amount of power to drive them quickly. Of course, this depends on how you are interfacing with the DAC, but most serial protocols implemented in MCU peripherals will not be able to transfer data that fast. Typically, they require at least two bus clocks for each bit that is sent, and they tend to have additional overheads too.

    If the DACs are on-chip, you may not have this problem. On-chip DACs often don't have a sufficiently high precision for some applications, but you said that you only require 8 bits so if they're fast enough they may be worth considering. Also bear in mind that many on-chip DACs can be made to run faster at the cost of some precision, so a 10-bit DAC that isn't quite fast enough may be configurable to allow it to run faster if you accept that the lowest two bits will not be correct. This is certainly the case with the ADCs on NXP's LPC devices as I've used them before, and DACs are often similar in implementation to ADCs.
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