The Clock Generation Subsystem: The Heartbeat of the Image Sensor |
Executive Summary |
This article provides a comprehensive exploration of the clock generation subsystem for image sensors in barcode readers, focusing on CCD (Charge-Coupled Device) and CIS (Contact Image Sensor) technology. We examine how precise, multi-phase clock signals are the lifeblood of these sensors, orchestrating the transfer of charge from pixel to pixel and ultimately to the output stage. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Analog Devices, Texas Instruments, and Altera. We explore the fundamental requirements of CCD timing, the role of dedicated clock drivers, the integration of timing generation in analog front ends like the AD9929 and AD9994, and the use of programmable logic devices for flexible timing design. The article covers both the fundamental principles and the practical implementation details that make modern image sensor readout possible. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting clock generation systems for barcode reading applications. |
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Chapter 1: The Heartbeat of the Image Sensor |
A CCD or CIS image sensor is not a passive device. It is a dynamic, clock-driven machine that requires a precise sequence of voltage pulses to operate. These pulses, known as clock signals, control the movement of charge through the sensor, orchestrating the transfer of charge from one pixel to the next and ultimately to the output amplifier. |
Without these clock signals, the sensor is just an array of photodiodesa collection of light-sensitive cells with no way to read out the information they have captured. The clock generation subsystem is the heartbeat of the image sensor, providing the timing and drive necessary to transform a static image into a time-varying electrical signal. |
The clock generation subsystem must meet several demanding requirements. The clock signals must have precise timing relationships (phase relationships) to ensure correct charge transfer. They must have sufficient voltage swing to fully move the charge. They must have fast rise and fall times to operate at high speeds. And they must be generated reliably over temperature and supply voltage variations. |
A research paper on CCD drive technology explains the key trade-off: 'When the drive capability is insufficient, the drive clock will produce attenuation, thereby reducing the charge transfer efficiency. In serious cases, the detector will fail to work properly' . This is why clock generation is not a trivial matterit is central to the sensor's performance and reliability. |
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Chapter 2: The Basic CCD Clocking Requirements |
CCD sensors require multiple clock phases to transfer charge. A typical two-phase CCD requires complementary clocks (phase 1 and phase 2). A four-phase CCD requires four overlapping clocks. The specific requirements depend on the sensor design. |
The key clock signals for a linear CCD include: |
Transfer clock (SH): This signal controls the transfer of charge from the photodiode array to the shift register. When SH is high, charge is transferred from the photodiodes to the shift register. When SH is low, the shift register is isolated, and the charge is read out. |
Shift clocks ([w]1, [w]2): These signals control the movement of charge along the shift register. The clocks are typically complementary ([w]1 and [w]2 are opposite phases) and overlap to ensure smooth charge transfer. |
Reset clock (RS): This signal resets the output node before each pixel is read. The reset operation removes the previous pixel's charge from the output stage, preparing it for the next pixel. |
Clamp pulse (CP): This signal clamps the video output to a reference level, removing the DC offset. |
A design paper on the TCD1208AP CCD describes the requirements: 'The device requires four driving signals: transfer signal SH, reset signal RS, two-phase shift clocks F1 and F2. The timing diagram shows the strict relationships that must be maintained between these signals' . The paper also notes that 'SH must remain high for a minimum of 1000ns to ensure complete charge transfer' . |
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Chapter 3: CCD Clock Drivers |
The clock signals generated by the timing generator are not directly connected to the CCD. The CCD's clock inputs are large capacitive loadsoften hundreds or thousands of picofaradsand the timing generator cannot drive such loads directly. Clock drivers are required to boost the voltage and current of the clock signals. |
A description of the CCD clock board for the Pyxis camera provides a detailed example of clock driver design. High-speed MOSFET clock drivers were used to generate all the clocks. The logic input to these clock drivers is level-shifted using a capacitive coupling scheme. The advantage of this technique over the traditional transistor level shifter approach is that 'the quiescent power dissipation is much lower, and the signal rise/fall time is faster' . |
The driver selection is critical: 'The selected clock drivers should easily be capable of driving the load capacitance of the KAF1602 chip without increasing the vertical clock cycle time' . The clock driver must provide the correct voltage swing and current to charge and discharge the CCD's input capacitance quickly. |
For the horizontal clocks, a driver with a single inverting and a single non-inverting output was used. This allows the complementary phases to be generated from a single input. 'The cross-over of the clocks can be precisely adjusted by trimming resistor R48 (this introduces a small delay between the two clock inputs). The useful adjustment range is several ns' . |
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Chapter 4: Timing Generation with CPLDs and FPGAs |
The timing generation function is often implemented in a programmable logic devicea CPLD or FPGA. These devices can generate the complex, multi-phase clock sequences required by CCD sensors with precise timing and minimal external components. |
A design paper on a linear CCD driver explains the advantages: 'With the CPLD-based solution, the circuit has changed from a complex design to a design that is mainly implemented using a single ispLSI1016 chip. The system integration has been improved, and the system's anti-interference ability and stability have also been enhanced. The design and debugging cycle has been shortened to hours' . |
The CPLD implementation divides the timing generation into functional modules. For the TCD1208AP CCD, the modules include a counter for controlling the integration time, logic for generating the shift clocks, and logic for generating the reset and clamp pulses. The counter allows the integration time to be adjusted by changing the counter's maximum count value. |
Another design paper describes a CPLD-based driver for a 1024-element CCD. The driver uses a CPLD to generate the transfer, shift, reset, and sample-hold clocks. The paper notes that 'CPLD has advantages in integration, speed, and reliability' over other methods . |
A design paper on a 7500-element CCD driver describes an FPGA-based implementation with adjustable exposure time. The design uses a modular approach with separate VHDL modules for each function. The integration time is controlled by three digital inputs, allowing eight different integration times to be selected . |
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Chapter 5: The Integration of AFE and Timing Generator |
Modern CCD signal processors integrate the analog front end (AFE) and the timing generator on a single chip. This integration reduces the component count and simplifies the system design. |
The Analog Devices AD9929 is a highly integrated CCD signal processor that includes a complete analog front end and a full-featured, programmable timing generator. The timing generator is based on a 'Precision Timing' core with a resolution of 0.58 ns . The AD9929 includes on-chip horizontal and vertical clock drivers, allowing it to connect directly to the CCD sensor. |
The AD9929's timing generator provides all the necessary CCD clocks: RG clock, horizontal clocks, vertical clocks, sensor gate pulses, a substrate clock, and a substrate bias pulse . The operation is programmed via a three-wire serial interface. |
The AD9994 is a similar device that supports up to 12-phase vertical clocks, allowing it to drive advanced CCDs with multiple readout modes. The Precision Timing core in the AD9994 has a resolution of less than 600 ps . This level of precision is essential for high-speed CCD readout. |
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Chapter 6: The Counter-Based Timing Design |
Many CCD timing generators are based on a counter that counts the clock cycles. The counter determines the timing of the various clock signals, and its maximum count determines the integration time (the exposure period). |
A design paper on a CPLD-based CCD driver describes the counter-based approach. The counter is a 13-bit counter that counts up to a maximum value. When the counter reaches the maximum value, it generates the transfer pulse (SH) and resets. The maximum value determines the integration time. |
The counter output is also used to generate the shift clocks. A control signal is derived from the counter that enables the shift clocks during the readout period and disables them during the integration period. This ensures that the shift clocks are only active when charge is being read out. |
The design paper notes that 'adding a delay circuit to the program can extend the integration time further, to meet the needs of applications requiring particularly long integration times' . |
Chapter 7: The Role of the Master Clock |
The master clock is the reference for all the timing signals. It is typically a crystal oscillator or a high-frequency clock source. The master clock is divided down to generate the various clock signals. |
For a 1 MHz shift clock, a master clock of 4 MHz might be used, divided by 4. The reset clock might be generated from the 1 MHz clock, divided by 2. The transfer pulse might be generated by counting a certain number of shift clock cycles. |
A design paper on a linear CCD driver describes a system with a 4 MHz master clock. The 4 MHz clock is divided by 4 to produce the 1 MHz shift clocks. The 1 MHz clock is divided by 2 to produce the 500 kHz shift clock for the readout. The transfer pulse is generated by a counter that counts up to 1106 cycles of the 1 MHz clock . |
The master clock must be stable and accurate. Any jitter or drift in the master clock will affect the timing of the CCD clocks, potentially degrading the image quality. |
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Chapter 8: The Level-Shifting Problem |
CCD clock signals must have a voltage swing that is sufficient to fully move the charge. For many CCDs, the clock high level is 5 volts, and the low level is 0 volts. For other CCDs, the clocks may need to swing to -5 volts or +10 volts. |
The Pyxis CCD board description describes a capacitive coupling scheme for level shifting: 'Since the logic input to these clock drivers is referred to the lowermost supply, the 5V CMOS/TTL input signal is level-shifted using a capacitive coupling scheme' . This technique allows the clock driver to generate signals with a different voltage swing than the logic input. |
The capacitive coupling scheme has a disadvantage: 'There is a finite amount of time in which the clock can remain in the (non-quiescent) level which is governed by the RC time constant of the coupling-capacitor/tie-up(down)-resistor combination' . This means the clock cannot be held in a non-quiescent state indefinitely. |
An alternative approach is to use level-shifting circuits based on transistors or dedicated level-shifter ICs. These circuits provide static level shifting, allowing the clock to be held in any state for an arbitrary duration. |
Chapter 9: The Cross-Over Control |
For complementary clock phases (such as [w]1 and [w]2), the cross-over pointthe point where the two clocks cross during a transitionis important. If the cross-over is too early, charge may not be transferred correctly. If it is too late, the clock period may be too short. |
The Pyxis CCD board description discusses the cross-over control: 'The cross-over of the clocks can be precisely adjusted by trimming resistor R48 (this introduces a small delay between the two clock inputs). The useful adjustment range is several ns' . |
The designer notes: 'Optimally, the clock timings should be set so that the complementary phases cross at their 50% levels' . This ensures that the charge transfer is balanced and efficient. |
The cross-over control is typically achieved by inserting a variable delay into one of the clock paths. The delay can be adjusted during testing to achieve the optimal cross-over point. |
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Chapter 10: Clock Generation with Dedicated ICs |
While CPLDs and FPGAs are common for timing generation, dedicated clock generation ICs are also available. These ICs integrate the timing generator, clock drivers, and sometimes the analog front end on a single chip. |
The Analog Devices AD9929 and AD9994 are examples of such dedicated ICs. They integrate the timing generator and the AFE in a single package . The timing generator is programmable via a serial interface, allowing the clocks to be configured for different CCDs and operating modes. |
The AD9929 includes the Precision Timing core, which provides high-resolution timing control. The core allows the user to program the precise timing of each clock edge, enabling precise control of the CCD's operation. |
Dedicated ICs have the advantage of being optimized for the specific requirements of CCD timing. They are also typically more reliable and easier to use than discrete implementations, as the critical timing is controlled by the IC's internal design. |
Chapter 11: The Time-Varying Clock Load |
The load on the CCD clock drivers varies with the operating frequency. At higher frequencies, the capacitive load draws more current, and the clock drivers must deliver more power. This can cause the clock signals to distort. |
The high-frame-rate CCD research paper explains the problem: 'The CCD drive clock's load is a large capacitive load. When the charge transfer occurs, the pixels are charged and discharged rapidly, and the load current is relatively large. When the drive capability is insufficient, the drive clock will produce attenuation' . |
The paper derives a model for the clock driver load and shows that the clock rise time is determined by the capacitance and the drive current. To achieve fast rise times, the driver must be able to deliver a large current. |
At high frame rates, the clock period is short, and the rise time becomes a significant fraction of the period. This can reduce the charge transfer efficiency and degrade the image quality. |
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Chapter 12: The Clock Generation in a Complete Scanner System |
In a complete barcode scanner system, the clock generation is typically controlled by a system controller that also handles the decoding and communication. The timing signals are generated by a CPLD or FPGA based on commands from the system controller. |
A patent on a card reader describes a system where a processor emits a master clock frequency, which is then converted to the CIS clock signal by a CPLD . The CPLD also generates the start pulse (SP) to initiate a scanning period. |
The patent explains: 'The CPLD is a programmable logic device that can be configured to generate the sequence signals required by the AFE and the CIS' . This configuration allows the same hardware to be used with different CIS sensors or different scanning modes. |
The system controller can also adjust the timing parameters by reprogramming the CPLD. This provides flexibility for different barcode densities and scanning speeds. |
Chapter 13: Clock Generation for CIS Sensors |
CIS sensors have similar timing requirements to CCD sensors, but they often have fewer clock phases. A CIS module typically requires a start pulse (SP), a clock signal (CP), and a power supply. The start pulse initiates the readout, and the clock signal shifts the data out. |
The card reader patent describes the clock generation for a CIS: 'The SP signal is emitted from the processor and received by the CIS component module to initiate a scanning period. The CLK signal is generated by the CPLD and drives the shift register in the CIS' . |
The CIS clock frequency is typically determined by the required readout speed. A higher clock frequency allows faster reading but may increase the power consumption and noise. The clock frequency must be chosen to balance these trade-offs. |
The CIS's internal timing is typically simpler than a CCD's, as the sensor has fewer phases and the output is digitized on-chip. This simplifies the clock generation requirements. |
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Chapter 14: The Power Consumption Consideration |
Clock generation consumes a significant amount of power in a barcode reader. The clock drivers must charge and discharge the CCD's input capacitance, and this requires current. The power consumption is proportional to the frequency and the capacitance. |
The Pyxis CCD board description notes that the clock driver's quiescent power dissipation is low, but the dynamic power dissipation is significant . The dynamic power is calculated from the capacitance, the voltage swing, and the frequency. |
The high-frame-rate CCD research paper provides a concrete example: 'The load of the light-sensitive area clock is as high as 7.2 nF. At a transfer clock frequency of 7.49 MHz, the clock driver must deliver a significant current to charge and discharge this capacitance' . |
Power consumption can be reduced by reducing the clock frequency, using lower-voltage clocks, or using more efficient clock drivers. However, these measures may affect the image quality or the readout speed. |
Chapter 15: The Clock Generation and Signal Quality |
The quality of the clock signals directly affects the image quality. If the clock signals have excessive jitter, the readout timing will be uncertain, and the image will be noisy. If the clock signals have slow rise/fall times, the charge transfer will be incomplete, and the image will be blurred. |
A research paper on CCD drive technology explains the importance of clock quality: 'When the drive clock waveform has attenuation, the clock's rise time increases. The period for charge transfer to occur is reduced, and the charge transfer efficiency will be reduced. This can cause a reduction in the output signal and a large amount of residual charge, which reduces the CCD's MTF' . |
The clock quality is influenced by the clock driver design, the PCB layout, and the power supply noise. Careful attention to these factors is required to achieve the desired image quality. |
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Chapter 16: Summary The Clock Generation Subsystem in Perspective |
The clock generation subsystem is the heartbeat of the image sensor. It provides the precise, multi-phase clock signals that orchestrate the transfer of charge from pixel to pixel and ultimately to the output amplifier. Without these clock signals, the sensor is just an array of photodiodesa collection of light-sensitive cells with no way to read out the information they have captured. |
We have examined how different companies and technologies have approached the challenges of clock generation: |
Analog Devices provides the AD9929 and AD9994, highly integrated CCD signal processors with programmable timing generators. The Precision Timing core provides timing resolution as high as 0.58 ns, enabling precise control of the CCD clocks . |
The Pyxis CCD project demonstrates the practical design of a clock board for a scientific CCD camera, with high-speed MOSFET drivers, capacitive coupling level shifting, and precise cross-over control . |
A CPLD-based CCD driver design uses an ispLSI1016 device to generate the timing signals for a 2160-element CCD. The design divides the timing into functional modules, with a counter controlling the integration time . |
An FPGA-based CCD driver design uses an Altera EP1K30 device to generate the timing signals for a 7500-element CCD. The design uses VHDL to describe the timing generation, with adjustable integration time controlled by digital inputs . |
A research paper on high-frame-rate CCD drive technology analyzes the clock driver load and shows the relationship between driver capability, clock rise time, and charge transfer efficiency . |
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The key lessons from our exploration are: |
Clock signals are the lifeblood of the image sensor. The timing signals control the movement of charge and determine the quality of the output signal. |
Clock drivers must drive capacitive loads. The CCD's clock inputs are large capacitive loads, and the clock drivers must be able to charge and discharge these capacitances quickly. |
Timing generation can be implemented in CPLDs or FPGAs. These devices provide the flexibility to generate complex timing sequences with precise control. |
Integrated AFEs simplify the design. Devices like the AD9929 and AD9994 integrate the timing generator with the analog front end, reducing component count and simplifying the design. |
Clock quality affects image quality. Poor clock quality can cause incomplete charge transfer, resulting in a reduced output signal and degraded image quality. |
Power consumption is a key consideration. The clock drivers consume significant power, and measures must be taken to reduce power consumption while maintaining clock quality. |
In the end, the clock generation subsystem is a testament to the importance of precise timing in image sensor readout. It is the heartbeat that keeps the sensor alive, and its quality determines the quality of the image. The art of clock generation lies in the careful balance of precision, power, and cost, creating a subsystem that delivers clean, reliable clock signals to the image sensor. |
cs@easiersoft.com