Universal Flash Storage (UFS2.0)

Are you buying a mobile or tablet? One of the major specifications that you look at is the storage. The need for memory in embedded products is vital. So, memory is a must in embedded products. We always here of eMMC card that is used in these systems. The memory that we are talking about here is NAND type and this is the type of memory that is used for storage as it is the cheapest memories available. The newest memory interface that the latest processors have to connect memory interfaces is UFS.

UFS is a universal flash storage interface in processors for connecting external memory. UFS is a next generation JEDEC flash memory standard. UFS is a successor to eMMC standard that is commonly used. The removal memory in the embedded systems are interfaces using this interface. The previous eMMC standard uses parallel interface where as the UFS uses serial differential interface for connectivity. The serial differential interface is basically a LVDS signalling method used. On most of the latest mobile processors, you find the UFS interface. The UFS is basically a full-duplex interface. As the interface is full-duplex, this will have separate lines for read and write making it the fastest interface.With UFS we can say that the speed/performance of SSD is combined with storage capacity of eMMC. The speed of UFS based memories can be up to 1200MBps.So, we can achieve the same speed with UFS as well.

The present version of UFS is 2.0. 

Why is UFS preferred?

1. The sequential read/write speeds are high (Faster response times)

2. Lower power consumption.

3. Higher storage bandwidth (Memory density)

4. Lesser space required on PCB for routing

5. Bi-directional transfer

6. The commands are queued which helps for parallel processing

7. Lower latency than eMMC interface

The latest Snapdragon 820 processors have the UFS2.0 interface.

High speed Designs - Part 2

Eye Diagram in communications field is one of the indicator of channel performance. The quality of the signal and in turn channel can be explained by the eye diagram. the eye diagram is a source to analyze the high speed designs. An eye diagram is formed by overlaying of number of bits that are transmitted across the channel. An eye diagram is a pattern that is formed in an oscilloscope which resembles the shape of an eye. If you have validated or performed any certification of high speed interfaces like PCIe, USB, etc., then you know well that eye diagram is very crucial to such analysis. In such certifications/validation a fixed data pattern is transmitted from the scope and received back. Example of such patterns is PRBS. Eye diagram basically is a voltage/time samples of the actual data. the eye diagram is representation of some million samples of data patterns captured. Oscilloscope is to be used in persistence mode to capture the sequence of samples.

What can be measured using an eye diagram?

1. Rise Time

2. Fall Time

3. Jitter

4. Over shoot

5. undershoot

6. Bit error rate

7. Cross talk

8. Inter symbol interference

9. Signal-to-Noise Ratio

What do we do/How do we analyze using eye diagram?

1. When we are measuring a interface performance, like for example, USB, we have to determine the acceptable levels of signal that help for exact reproduction of the signal at the receiver. For this we draw a small diagram called eye mask. If the generated eye diagram is well beyond this eye mask, then channel is performing well and doesn't have any signal integrity issues. If any of the eye diagram overlaps eye mask then there is a serious issue in the channel.

2. An eye opening (so called height of the eye or peak-to-peak value) is an indication of the channel performance. If an eye diagram generated is observed to be closed, then there is signal integrity issue in the channel.

3. The overshoot and undershoot indicates the impedance mismatch that is present in the channel because of which reflections occurred.

4. The eye width indicates the unit interval, and is an indication of any jitter present in the channel.

The basic test setup for generating eye diagram is:

The following image is a capture of eye diagram. Snapshot is taken from edn.com/keysight.com to give a view of the eye diagram analysis and capture.

Definitions used in eye diagram:

One level: The highest level (logic high voltage) in a peak-to-peak representation of the signal

Zero level: The lowest level (logic low voltage) in a peak-to-peak representation of the signal

Amplitude: The difference between one level and zero level.

Eye Height: Indication of eye opening. Under ideal conditions, Amplitude is equal to eye height. 

Eye crossing: Indication of jitter that is present in the channel.

Disadvantages of eye diagram:

1. Eye diagram can only judge but can't guarantee the channel performance.

2. Eye diagram is generally captured with a standard data pattern but in real time the pattern can vary.

3. Eye diagram can't determine the actual cause of the signal degradation in the channel.

Analog Circuits - Logarithmic Amplifier

There are various applications of op-amps. One of the important circuits using op-amp circuits is logarithmic amplifier. In logarithmic amplifier, the output is natural log of input signal.  the input signal can be a simple voltage. The basic equation that describes the logarithmic amplifier output is,

Vout = K * ln (Vin), K is gain of logarithmic amplifier

As we are saying logarithmic, this op-amp circuit is non-linear. So, a logarithmic amplifier converts linear voltage to non-linear. The main advantage of logarithmic amplifier is the dynamic range compression. So, if we have a very large dynamic range, it can be manipulated using a logarithmic amplifier.

The basic logarithmic amplifier circuit with diode is:

The basic electronic devices that help achieve logarithmic function are diodes and transistors. In a transistor, Vbe is related to Ic in a logarithmic way.

Output = -V_T ln(Input/I_s*R)

V_T  Thermal Voltage of the diode

 I_s    Saturation current in the diode

The reverse saturation current of the diode is temperature dependent.

Another logarithmic amplifier circuit with transistor is:

Some important points about logarithmic amplifier:

1. The gain of an ideal logarithmic amplifier approaches infinity as input approaches zero.

2. 

3. Change in output of logarithmic amplifier is the function of change in input voltage

3. Intercept voltage: The voltage at which the logarithmic value leads to zero. 

4. For smaller inputs, logarithmic amplifiers behave linear. The relation between input and output is linear.

Circuit by explanation: If i want to product of two analog signals, for suppose X and Y, we can do it using the below equation,

X * Y = Loginv(LOG(X)+Log(Y))

The above equation when represented in the form of block diagram, 

Application of logarithmic amplifiers:

1. Industrial circuits

     a. Process control

2. RF circuits

     a. Compression and decompression

     b. True RMS detection

Some vendors of logarithmic amplifiers used across different verticals of the industry are:

Texas Instruments - ADL5513 (Example of ic that is used in RF transmitter circuits)

Analog Devices - AD8307 (used in network and spectrum analyzers)

Maxim Integrated - MAX4206 (industrial applications)

Note: The reverse of logarithmic amplifier is anti-log amplifier where the non-linear signal is converted to linear signal.

High speed Designs - Part 1

A signal can be considered high speed, if the signal frequency that need be transmitted approaches 100MHz. There are many  parameters to decide if a signal can be considered high speed or not but let us for now consider 100MHz or greater frequency as high speed signal. When we talk in terms of bit rate it comes to 100Mbps if each cycle carries a single bit. as a straight forward definition, a signal can be treated as high speed if the speed is the major reason for the loss of the signal. Putting it the other way, the minimal frequency at which the signal starting degrading if improper care is not taken while routing it. All the frequencies above it are known as high speed frequencies. In another terms, frequency at which all the line parameters (R,L,C) are to be considered for analyzing the signal or the channel need to treated as transmission line can be called high speed signals.

                Routing a high speed signal is not so easy. There must be appropriate care taken starting from the PCB material selection to final routing stage. this implies from component selection, placement, stack up and routing each and every stage is critical in determining the quality of the board. We should take care that no signal integrity issues come up on the board. So, whether it is a high speed or low speed there are few factors that contribute to the loss of a signal on the PCB. The below are the list of items that are major contributors of loss in a PCB:

1. Package loss

2. Connector loss

3. Losses due to PCB traces (channel loss)

4. Losses due to via

The parasitics that the ic package introduces is the major contributor of losses. A package need to be treated as electrical model as such a model behavior need to be used in simulations. Consider, the electrical model of any ic (Ex: .ibis models), they list the R,L,C of the package pins. The Power distribution network of these ics and the modelling of these package connectivity is very crucial in signal integrity simulations.

Connectors are a must on any PCB to connect the board to the external world. Connectors are not always ideal. The characterization of the connector determines the accuracy of loss calculation. It is always preferable to select a low loss connector for high frequency designs.

The Channel loss or the PCB trace loss is due to 2 main factors: PCB material selection, PCB routing. The PCB material (dielectric material) is the crucial selection factor for high speed board designs. It is preferable to have a low loss dielectric for high speed signals. Also, narrower the PCB traces, higher the losses. Have as much wider trace as possible for lower losses. Again there is a trade-off between PCB material to be used, impedance of the board, width of the trace. The designer needs to make a careful selection to have a lowest loss signal.

Vias are one of the major loss on the PCB. Vias are the main reason for discontinuities on the board. The via type selection is vital at the preliminary stages of the high speed design. The types that can be used are blind via, buried via, micro via, differential vias.  Back drilling is used in some boards in case of micro vias to reduce losses. The via length in these cases is the major loss contributor. The return path for the signal path which has vias is very crucial for maintaining the integrity of the signal. If the vias are not laid properly, the introduced noise may cause substantial damage to the end product in use causing endless breakdowns. The via annular ring need to be designed after considering the amount of loss that is tolerated.

If you are working on high speed board, these PCB factors are very important and some designers calculate the loss budget with these parameters before proceeding with the manufacturing. Loss budget in these cases is the sum of the loss parameters (Connector loss, via losses, PCB trace losses, package parasitic losses) that we have talked of here.

Soldering Machines

This post is written on the request of one of the reader of our blog. thanks for the mail, it encourages us to write more articles when readers request for more.

Soldering is a process which is used to join two materials using a joint. The joint usually called a soldering material which has a lower melting point. Soldering machines are the tools used to do the soldering process. One more material used during the soldering process is the flux. Flux facilitates the soldering process. Soldering machines come in various shapes and sizes. It can be in the form of a gun or it can be in the form of a handle. A removable heat element at the tip of the machine facilitates soldering process. 

Temperature controlled or not?

The soldering machines are either temperature controlled or may be of fixed temperature. The temperature controlled soldering machines are a big set up and are commonly named soldering station. the soldering station is a combination of heat element, handle and a power supply regulation circuit. Soldering stations do have a display to read the tip temperature. A thermistor on the tip enables to read the temperature which is displayed on the screen.

Soldering iron power rating.

           Soldering machines are available in various power ratings. A machine with high power rating should not be used for minute joint soldering. That may damage the electronic components. Where as low power machines should be used for minute joints. the power ratings of the soldering machines range from 10-50W.

Soldering iron tip

Generally called a bit which can be detachable and comes in various sizes. The size is determined by the size of the joint that must be soldered. There are various tip sizes available. A slanted tip at the edge may have a more heat holding capability where as straight bits have less heat holding capability.

Various vendors who provide soldering machines:

METCAL

SOLDRON

ADVANCETECH

MAX TECHNOLOGY

METROQ

Wetting current (Sealing current)

Switches/Relays require some initial current to break the oxidation layer that is formed across the contacts. Supplying this amount of current helps the contact form reliably. The current supplied heats the oxide layer. This amount of current is obviously an additional burden on your power circuit but is the minimum current required to form a contact. Oxidation generally happens in humid conditions where the oxide layer is formed across the joints. The oxidation can generally be treated as resistance when you speak as an electrical engineer. The scenario of wetting current can be seen in the case of switches where you provide the stimulus but still doesn't turn up, where little more current would have been required there. Generally, in the humidity tests, these kind of issues come up on the board. The amount of whetting current depends on the material that is used for the contacts. For example, a gold plated contact may form a reliable contact with less wetting current. the two generic circuit parameters you may across in this scenario are punch through and stand-off voltage. Stand-off voltage is the minimum voltage that need to be provided for the oxide layer to break through. 

Dithering in circuits

Are you designing a board which requires agency certifications? Do you want to control any EMI/EMC concerns that are arising out of power supply section. One way to address your concerns is dithering of power supply. You might have taken proper concerns in layout but sometimes the power supply may be the source of noise in spite of extreme care taken in design. Dithering is nothing but a small electronic circuit to control the switching frequency of the power supply. The frequency spectrum of the oscillator can be spread to increase the band of frequency. Frequency spread meant distributing the total energy among the multiples of fundamental frequency. The dithering circuit is a simple op-amp circuit controlling the voltage into one of the frequency determining pin of the switching power supply. The voltage on this pin of switcher determines the frequency of operation of the switcher. Have you ever used a spectrum analyzer to probe a frequency generator in the circuit? If you try to probe the signal, you see a peak at the fundamental frequency of the switcher but if the signal is measured with dithering applied we can see the signal distributed over a band of frequencies. This technique can also be said as spread spectrum where the frequency is spread over a band.