Abstract:
There is provided a power supply monitor circuit comprising a positive supply monitoring input, a negative supply monitoring input, and a voltage divider connected between the positive and negative supply monitoring inputs and having an intermediate node providing a potential intermediate those on the positive and negative supply monitoring inputs. A sampling circuit is connected to sample the potential at each of the positive supply input the negative supply input and the intermediate node. Also provided is a method of monitoring noise on a power supply whereby phase information from those samples is derived about the noise signals at the two said points.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention provides a power supply monitor and is intended to be capable of assessing the noise on a supply line both with respect to amplitude and frequency content. The circuit is based on a published paper, (Elad Alon et al. VLSI Symposium, 2004) but includes a number of enhancements.  
       SUMMARY OF THE INVENTION  
       [0002]     The present invention provides a power supply monitor circuit as defined in the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]     An example of the present invention will now be described with reference to the accompanying drawings, of which:  
         [0004]      FIG. 1  is a block diagram of a power supply monitor circuit according to the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0005]     In the circuit of  FIG. 1 , a particular point on the positive supply within the circuit to be monitored is selected using a multiplexer I (ASEL_MUX) and this potential is applied to a potential divider  2 . A sample &amp; hold function  3  stores the value at one of three sample points for a specific moment in time. The three points that can be sampled are the selected positive supply point VDD  4 , a negative supply sample point VSS  5 , and the mid point  6  of the voltage divider  2 . Preferably the voltage divider provides at the mid point (VDD+VSS)/2, but other ratios are possible. The point sampled is selected by switches  7 . In the example of  FIG. 1  only one negative supply sample point is shown.  
         [0006]     The voltage stored by the sample and hold function  3  is applied to a voltage controlled oscillator VCO  8 , which, of course, by definition, causes the VCO frequency to be modified. The frequency of the VCO  8  is evaluated using a counter  9  for a fixed period of time. Conveniently the period is fixed with respect to the (or a) clock used to clock the circuit that is being monitored, but any other signal providing an interval of time would do. In the example of  FIG. 1 a  low speed clock  10  is shown provided by a divider  11  which divides a “byte clock”, which provides a suitable length interval. (Clocks are termed “byte clock” when data is converted from serial to parallel streams e.g. by a shift register and the “byte clock” is the clock used to clock the parallel bytes output by the shift register, whereas the “bit clock” is the clock used to clock the serial data stream.) Registers  12  are provided to receive the count from the counter  12 . Control of the circuit of  FIG. 1  is provided by a control circuit STCI  13 , which inter alia controls the counter  9  to measure the frequency of the VCO  8  and scans the recorded counts in the registers  12  out to an external analyzer computer  14 . The analyzer computer  14  may also command the monitoring circuit  14  to perform its monitoring functions.  
         [0007]     After calibration, the output of the counter  9  can be related to the sampled voltage. The calibration process is described later below. Note that the sample and hold function  3  and the VCO, of course, have a signal ground against which the level of the potential they measure is judged. In general this ground is not the same ground as the VSS  5 , which of course is measured by the circuit.  
         [0008]     Advantageously the monitor circuit of  FIG. 1  is provided as part of the integrated circuit that is to be monitored. This reduces the impedance of the lines connecting the point sampled to the sample and hold function  3 . The sample and hold function  3  when sampling charges a capacitance, so therefore siting the monitor circuit in the integrated circuit reduces the RC time constant and preserves high frequencies in the signals from the monitored points so that they can be measured.  
         [0009]     By taking repeated samples at uncorrelated moments in time, a picture of the amplitude variation of the supply can be built up. Since the waveform of the potential of the point sampled is sampled at random points in time eventually the samples taken will cover all the range of the values taken by the waveform. This therefore gives a measure of the amplitude of that waveform.  
         [0010]     The technique permits the evaluation of both the VDD and the VSS at a specific point within a chip. Evaluation of these two points provides amplitude information of each point individually, but without phase information, the relative amplitude (VDD-VSS) is not available. The purpose of the potential divider is to provide the necessary phase information by measuring the mid point (VDD+VSS)/2. If VDD and VSS are in anti-phase then the noise at the mid point will be zero. (Strictly of course the noise at the midpoint will be zero if the VDD and VSS are in antiphase and have the same amplitude. If they are in antiphase and have different amplitudes then (for a single frequency) the amplitude of the noise at the mid point will be half the difference of the amplitudes of VSS and VDD.) If they move in phase then the noise at the mid point will not be attenuated.  
         [0011]     Frequency information about the noise is obtained using an auto-correlation technique. Two samples are taken from the same point in the circuit but shifted in time by a controlled amount. Frequencies of interest are evaluated with a time shift controllable from 2 to 50 ns (with different shifts of course providing information about different frequencies in the noise. The sampling moment must be uncorrelated with any noise generating clocks on chip and is therefore initiated by a clock  16  from the control circuit STCI. A counter  17  working from the VCO  8  generates the time shift. The VCO  8  is free running with a reference voltage input. (The reference voltage is may be provided by a reference voltage circuit  18  provided within the chip or may, for extra accuracy by provided from a source external to the chip. For symmetry with the sampling of VDD etc (see below) the reference voltage is first sampled by a sample and hold circuit  19  within the sample and hold function  3  before being passed to the input of the VCO  8 .) Under the control of the reference voltage the VCO  8  produces a time interval that is not accurately controlled but can be calibrated.  
         [0012]     In order to take the two samples from the particular point on the power supply, two sample and hold circuits  20  and  21  are provided in the sample and hold function  3 . The sampling by one is initiated at the beginning of the count provided by counter  17  and the other at the end.  
         [0013]     A multiplexer  22  is used to select which one of the sample and hold circuits provides its sampled voltage level to the VCO  8 . During the sampling of the power supply the VCO input remains connected to the reference voltage so that counter  17  runs at the desired rate and sets the interval between the two samples of the particular point on the power supply. Once the sampling is complete, one of the sample and hold circuits  20  and  21  is connected to the VCO input and its resultant frequency is counted by counter  9  thereby measuring the voltage on that sample and hold circuit. Next the other one of the sample and hold circuits  20  and  21  is connected to the VCO input and its voltage is similarly measured.  
         [0014]     Note again that the measurement (i.e. the pair of samples) is repeated at random intervals so that the amplitude can be determined. (Consider for example sampling a frequency at two point separated by half the period of that frequency; depending on where the samples fall the difference between them can range from the peak to peak amplitude to zero.)  
         [0015]     In comparison to the present invention the Alon reference describes a technique for measuring VDD only, and has a restricted voltage range. This circuit covers a wide range of voltages from below VSS to above VDD, and the voltage is applied to a varactor that handles the wide input range. The wide range obviates the use of a buffer on the sample and hold capacitor so charge sharing techniques replace the buffer. Because the charge is shared between the sample and hold capacitor and the varactor, it is necessary to put the varactor in a known state prior to the charge share. This is done by opening the VCO loop to stop the oscillation and grounding one side of the varactor.  
         [0016]     A practical difficulty with this invention relates to the RC time constant resulting from the sample &amp; hold capacitor and the source impedance. The result is a frequency dependent attenuation that is different for each of the 3 monitor points. Furthermore, when both S&amp;H capacitors are connected to the monitor point, the attenuation is one value. After the first sample is taken and only one S&amp;H capacitor is connected, the attenuation adjusts to a new value. A novel technique to avoid having two attenuation values is now described.  
         [0017]     A single attenuation value is achieved by having only one capacitor connected at a time. For the simple case and the example circuit of  FIG. 1 , this would give the second sample a worst case of only 2 ns to charge from an unknown value and settle. This is not enough time with practicable impedance and capacitance values. The solution is to cycle several times with the 2 capacitors (i.e. sample and hold circuits  20  and  21 ) being alternately connected for the required period, say 2 ns. As a result, each capacitor is precharged to a voltage close to the final value before the final sample and hold period and time constant errors are reduced. This means that the sample and hold value contains some information from previous periods but this is beneficial as the frequency that is being analyzed will be reinforced and other frequencies will be attenuated. Only following the final period of charging are the two samples passed to the VCO  8  for their levels to be measured.  
         [0018]     Again the measurement is repeated at random to ensure that something approaching the amplitude of frequency is measured at one of the repetitions.  
         [0019]     Note that in the example circuit of  FIG. 1  the selected VDD point is measured with respect to the one particular VSS point  5 . If measurements of VDD with a different VSS point than VSS point  5  are required then this different VSS point can be connected with the multiplexer  1  which is then measured with respect to the VSS point  5 . This information together with measurements of VDD with respect to the VSS point  5  a can be then used to construct in the analyzer computer measurements of the VDD with respect to the said different VSS point. Alternatively the circuit could be arranged to select from one of multiple point on the negative supply; as indicated in dotted lines a multiplexer  15  can be provided to allow that.  
         [0020]     Sequence of Operation  
         [0021]     Signals  
                                       PD   Power down       EN   Enable the VCO to run       EN1   Connects S&amp;H capacitor 20 to the varactor. i.e. the VCO 8       EN2   Connects S&amp;H capacitor 21 to the varactor. i.e. the VCO 8       ENS   Connects S&amp;H capacitor 19 to the varactor. i.e. the VCO 8       CK1   Connects S&amp;H capacitor 20 to the input (i.e.           the selected one of VDD, VSS and their mid point)       CK2   Connects S&amp;H capacitor 21 to the input (i.e.           the selected one of VDD, VSS and their mid point)       CK3   Connects S&amp;H capacitor 19 to the reference.       CKOUT   VCO 8 output frequency                  
 
         [0022]     Initialization.  
         [0023]     Referring to  FIG. 1  the circuit is initialized by setting the following signals:- EN=lo, EN 1 =lo, EN 2 =lo, CK 2 =lo, CK 1 =hi.  
         [0024]     Calibration.  
         [0025]     To calibrate, first connect the input via the ASEL_MUX to an external voltage. Apply a range of voltages to the input for calibration.  
         [0026]     Keep CK 1  hi for a period long enough to charge the sample &amp; hold capacitor. Sampling on S&amp;H 1  occurs when CK 1  goes lo.  
         [0027]     Evaluate the sample as follows. Simultaneously make EN 1 =hi, EN=hi. Count the clocks at CKOUT for approx. lus using the divided byte clock or other slow clock. Shift out the count.  
         [0028]     Create table of input voltages vs clock count.  
         [0029]     Repeat with CK 2  to calibrate S&amp;H 2 .  
         [0030]     Taking a Reading.  
         [0031]     Set EN 3 =hi to connect the reference voltage to the VCO and cause it to generate a clock with a frequency not correlated with any other clock on chip. Then alternately set CK 2 =hi, CK 1 =hi, to connect S&amp;H 1 , S&amp;H 2  to net to be measured.  
         [0032]     Connect REF to a reference voltage. Keep CK 3  hi for &gt;10 ns. Then set CK 3 =lo. EN 3 =hi, EN=hi. Evaluate CKOUT frequency. Use CKOUT divided down to generate alternating CK 1  and CK 2  with an adjustable period from 2 ns to 50 ns and a minimum of 5 cycles.  
         [0033]     Stop the VCO by setting EN=lo. Wait &gt;10 ns.  
         [0034]     Evaluate the voltage stored on sample &amp; hold 1  by setting EN 1 =hi, EN=hi, count CKOUT for 1 us. Put count into register. Now reset by applying EN 1 =lo, EN=lo for &gt;10 ns.  
         [0035]     Evaluate sample &amp; hold 2  in a similar way. Set EN 2 =hi, EN=hi, count CKOUT for lus.  
         [0036]     Reset by applying EN 2 =lo, EN=lo for &gt;10 ns.  
         [0037]     Shift and tabulate CKOUT counts.  
         [0038]     Repeat for a range of CK 2 -CK 1  time shifts to reconstruct the spectral density.  
         [0039]     Repeat the procedure for VDD, VSS &amp; (VDD+VSS)/2 to get differential and phase information.  
         [0040]     While the invention has been shown and described with reference to preferred embodiments thereof, it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit and scope of the invention as defined by the appended claims.