AIBOU KAKASHI

Draft 2006/3/3

By Calvin Horng

Objective

This document is intended to be a hardware design guideline for those who have no experience on how to start from the scratch.

The pre-requisites

For those who want to start H/W design shall have basic concept on

(1)   Discrete components like resistors, capacitors, inductors …

(2)   Active device like diode, transistors…

(3)   Integrated circuit like 74 series logic device

(4)   Memory device like FLASH/SRAM/SDRAM…

(5)   Communication interface like I²C , SPI, UART or Ethernet…

(6)   Microcontroller or Microprocessor

(7)   For advance design, you might have to have skill on programmable logic like CPLD or FPGA.

Description

The following section will describe the above in simplified way.

(1)   Discrete components.

    For resistors, R=V/I, where V is the voltage across the resistor and I is the current flowing through the resistor. When selecting the resistor, some parameters are very important

(a)    Footprint: Take caution in height and the width.

(b)   Power rating: Most of the case, choose 2 x the power. For example if I=4mA and V=3.3V. The desired power rating shall be 2 x 4e-3 x 3.3 = 0.052 watt.

(c)    Material: Some design case will require the un-flammable resistor to meet safety requirement.

    For Capacitor, C=Q/V but in real world C = [(dQ/dt)/(dV/dt)]=[I/(dV/dt)]

Which means the current measurement must be taken using charging or dis-charging period.

        The capacitor are often used in some occasions,

(i)                 DC By-pass for better DC ripple performace

(ii)               DC-Blocking in signal path

(iii)             Filter design

When selecting the capacitor, some parameters are very important

(a)    Footprint: Take caution in height and the width.

(b)   Working voltage rating: Most of the case, choose 2 x the exact DC power across the capacitor. For example if the DC is 5V. The desired working voltage  rating shall be 2 x 5V = 10V.

(c)    Material: Different material has different Temp. coefficient.

(d)   Q value: It is important in high freq. design

(e)    ESR: Effective serial resistance will be somehow related to the Q value. The lower ESR is, then lower noise will be measured.

(f)    Capacitance: The value will be very important to determine the effectiveness in the filtering or pole.

    For Inductor/Coil, the formula depends on the physical structure and will not be described here.

        The inductor are often used in some occasions,

(i)                 Filter design

(ii)               Isolation for high frequency.

When selecting the inductor, some parameters are very important

(a)    Footprint: Take caution in height and the width.

(b)   Working current rating: If the current is too high and close to the max. rating, then there will be saturation in the inductor. When that happen, the effective inductance will be lower.

(c)    Material/Structure: Different material has different Temp. coefficient.

(d)   Q value: It is important in high freq. design

(e)    Inductance

The passive elements like R, L, C will be very important is the circuit design. Modern design will require smaller foot-print and better performance. But always keep in mind, there is no ideal R, L,C. All components will be a combination of all the three. What is meant is the resistor will behave like an inductor at higher frequency and capacitor might behave like an inductor in parallel with a resistor. Reading the performance data before using correct components will be very important esp. in high frequency design

(2)   Active device like diode, transistors…

    It is not often in modern design using real active discrete to build a circuit. But sometimes it is still needed. The diode and transistor has their own I-V curve, so when designing circuit, please find the correct load line and determine the correct working voltage and current.

        So the most of important parameter is the power rating. And make sure the selected device has room for over-temperature de-rating.

(3)   Integrated circuit like 74 series logic device

   When reading datasheet for the integrated circuit, take caution on the following items

(a)      Power supply

(b)     I/P AC/DC Characteristic

(c)      O/P AC/DC Characteristic

(d)     Timing

Most of the IC will require with power supply with low ripple noise to avoid in-correct logic operation. Good DC bypass to the power pin will be very important. When the Fan-out at the O/P is very high, pay attention to the current spec. If the current cannot drive the number of fan-out, correct the design.

(4)   Memory device like FLASH/SRAM/SDRAM…

        Memory is very important is modern embedded system design. Choosing the memory will based on

(a)    Size: The memory layout required by the S/W. This will determine the size of the memory

(b)   Bandwidth: The AC characteristic will determine how fast the memory device can be accessed and this will determine the computing performance.

(5)   Communication interface like I²C , SPI, UART or Ethernet…

        Read the corresponding datasheet for each interface will going to help. When determine which interface shall be used, please take following factor into consideration

(a)          Number of device in the bus:  Each interface has its limitation on the total number of the device connected to the bus.

(b)         Bandwidth: The access bandwidth will be also limited by different interface .

(6)   Microcontroller or Microprocessor

   Choosing a correct uC or uP is not a easy task. And need detail discussion with S/W designer.

   Basically a uC or uP shall have core and peripheral. Different architectures will give different system design and performance. Take the following factors in choosing devices

(a)    Computing power: Make sure the max. operating freq. and  how many clock cycle for instruction and data fetch. Most of the datasheet will give rough MIPS number.

(b)   Memory controller: Each uC or uP might have internal memory controller, when using the memory interface, some of the uC or uP might give DMA which will give better bandwidth for higher data transfer requirement. Some of the uC or uP might nit have memory control interface, external companion chip will be needed and raise the cost for implementation.

(c)    Peripheral: Some of the peripherals like UART or timer will be included in some of the uC or uP. The number of peripherals is also a determining factor.

In designing uC and uP, reading the datasheet or design guide is very important. When drawing schematic, keep the following checking list will help you from making mistake

(a)    Core power and I/O power: Modern uC and uP might have different core voltage and I/O voltage. Pay attention to the power sequence. Low ripple and enough current to the power connection is a must.

(b)   Boot-strap: Most of the uC/uP required external pull-ups or pull-downs at some dual-functional pins. Going through the datasheet and make sure each pin are correctly pull-up or pull-down and meet the functional requirement is very important. It will be also a good habit that the wordings are added aside the pin in the schematic drawing.

(c)    Power-on reset: Some of the uC/uP require reset pulse to be applied for some given time. Note the relationship between the reset and the power on curve. Some of the reset must be applied after system clock is stable. [This is synchronized reset].

(d)   Clocking: A correct clock to uC and uP is very important. Most of the moden uC/uP design has built-in PLL to raise frequency of the internal clock for core. Make sure the clock to the clock input has good wave-shape and good stability.

(e)    Addressing issue: When not running in 8-bit access, make sure the LSB of the address line is connected correctly. Most of the uC/uP might not have the bus switch, so the LSB of the address might not be A0.

(f)    Interconnection: Make sure the I/P and O/P sense are double checked when connecting the external device.

(g)   Some damping resistors in the bus lines: Most of the uC/uP is designed to drive high fan out. So putting the damping resistors in the O/P or bi-direcional pins will help when problem occurs in EMI tests.

(7)   For advance design, you might have to have skill on programmable logic like CPLD or FPGA.

     Sometimes, it is not easy to find appropriate off-shelf device to implement the functions needed. Thanks to modern programmable device, the lost functions can be implemented. Some key points are very similar to those points described in above section. But additional points are listed below

(a)    If possible, not use Dedicated pins for I/O:

(b)   Make sure the I/O power are correct for each I/O bank. In some case, different I/O bank are powered by different voltage to allow different logic level.

(c)    Leave the debug interface available.

(d)   Assign some of the test I/O with externally connected to test points. This is very important when the device is designed the first time

(e)    Add serial resistors at the O/P pins of the device if the output current can not be adjusted. This is for EMI reduction.

Caution must be taken since modern design on programming device will prefer language entry like Verilog or VHDL. Different coding will give different logic implementation and P&R result. Always look into the report files and understand the timing relationship. Bad timing constraint will give faulty logic result. Make good practice on the coding and using appropriate constraint files will help you from making time-wasting tuning.