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Powering Circuit Q's

Hello,

I am have some questions regarding the Schematics of the BF548 ezkit (attached). Specifically

the powering sections.

Firstly, a basic questions, what is the difference between the filled square and hollow

circle schematic symbol for the power buses? The page Power Page 1, for example,

on the bottom right shows the circuitry for the VDDEXT and 3.3V, shown as filled square

and hollow circle respectively. They are both 3.3V so what is the difference?

Looking now at Power Page 2, I don't really understand the power supply for VDDDR and

5V. What does the adp1823 chip setup do? I looked briefly at its datasheet but didn't

understand how it relates to the regulation of VDDDR and 5V (again the symbols are different?).

On the bottom left of this page there are two outputs for 5V_USB? Why?

Also, on the bottom right there are these symbols (MH4, MH5, etc) but they aren't connected

to anything? What are they for?

It's pretty evident there are lots of things that I'm confused with - the power circuitry has been

a bit of a hurdle. Is there an easier design for the power supply? Would really appreciate

any reference designs that have a simpler powering system.

Thanks to anyone who can be bothered to try and help. I know it is not exactly 'fun', but it

will be much appreciated.

Cheers...

A0206-2006_CUST.pdf
  • Hi,

    I have some more questions to add to this thread.

    Bypass Caps:

    An EE note (forget which one) described how it is best to put several different decades of capacitor values as bypasses to the

    dsp to raise the natural frequency of the power supply. The bf527 ezkit uses 10,0.1 and 0.01 uF caps whereas

    the bf548 just uses 10 and 0.01. Is there a specific reason for this? From the measurements, which is the best?

    Series Terminating Resistors:

    The bf548 has series terminating resistors on the PPI and DRAM lines, but (for example) the bf527 ezkit doesn't

    have these. Again, is there a reason? Are they necessary? Obviously the less components the better but also I

    wouldn't want to compromise signal integrity.

    Max DRAM size:

    I am a bit confused about max DRAM memory size. The bf548 data sheet says each external bank supports 256MB with a

    total of 512MB. However, with 13 bit addressing and 16bit data, isn't it 2^13 * 16 = 128MB? Just wondering

    where I am going wrong.

    Power Reg:

    Again comparing the bf548 and bf527 ezkit powering. They are very different, but essentially provide the same outputs. Is the

    bf527 better to use as reference because it is newer (I think?) or is that actually more to it.

    USB:

    The USBRSET pin is meant to (from bf548 datasheet) "connect to GND through an unpopulated resistors pad" - what is the difference between this and leaving it floating? If the pad is unpopulated then isn't it floating anyway?

    Thanks in advance for any assistance with these queries.

    Cheers..

  • Thanks for your interest.

    Bypass Caps:

    There is no easy answer to this.  The optimum frequency for a capacitor also depends on the package size and shape.  There is a theory that if the values are close to each other, it can create a new resonant frequency resulting in more power noise.  The system design section of the HRM has a list of references.

    Series Termination:

    Impedance matching resistors at the output circuits do 3 things.  First they reduce the voltage overshoot and undershoot seen at the receiver.  This prevents long term damage to the receiver.  Secondly they reduce reflections.  Un-damped reflected waves can be seen by the receiver as false edges.  The third advantage is reduced EMI.

    PCB trace simulation of overshoot using IBIS models predicted that the BF527 required external resistors where the BF548 did not require them.

    Your maximum SDRAM equation needs some work.  Remember that not all address lines are muxed during RAS and CAS.  There are also some Bank Select lines.

    Either the internal regulator or an external regulator can be used to generate the core voltage VDDINT.  This is often chosen for cost or flexibility.  The power management section of the HRM gives more detail.

    USBRSET allows for an external correction factor that is not required for any of our current silicon.  The system design section of the HRM has more detail.

    Hope this helps.

    _BobK

  • This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin