Microcontrollers contain a broad range of features due to the numerous end-use markets they address. Typical microcontrollers come in 8-, 16-, and 32-bit versions and commonly are used in consumer, automotive, energy management, and medical applications.
Consumer applications are very cost sensitive so it is common for these devices to be fabricated using older lithography nodes and tested in parallel at very high site counts. Leading-edge devices such as many of the 32-bit microcontrollers contain embedded flash memory and require newer lithography nodes to achieve the necessary performance at optimal cost. When applied to the automotive market segment, device quality is crucial.
The cost of test, quality of test, and the push for increased device capability create a unique set of challenges for ATE device power supplies. In addition to these factors, since many microcontrollers have long product lifetimes, ATE device power supplies need to support legacy microcontroller requirements as well as the requirements for leading-edge devices.
Production Floor Flexibility
A key component to the cost of test of microcontrollers is equipment utilization. Due to the wide mix of device types and test insertions on a production floor, the ATE device power supply needs to be flexible enough to provide a large number of outputs, varied maximum current capability, and a broad range of voltages.
Low pin-count microcontrollers may be tested from 32 to 128 sites in parallel. In many cases, the device will require two or three separate power supplies. For these test insertions, the ATE system needs to provide hundreds of power supplies with moderate power.
For complex high pin-count microcontrollers, four to eight sites are commonly tested in parallel. Each microcontroller being tested may require upwards of 10 power supplies. The purpose of testing with this number of supplies per device is to allow power domains within the device to be separated and tested individually.
Separation of the power domains is done for several reasons. For devices with embedded converters, it is common to separate analog and digital power supplies to minimize noise coupling in the converter. When a device has many cores, it can be important to measure the power consumption of the individual cores. Additionally, the ability to isolate faults in the device can provide valuable feedback to design and product engineers early in the device’s life cycle.
While many mobile microcontroller applications have evolved to use very low power to extend battery life, other applications require significant processing capability and, as a result, can be power hungry. The processing cores of these devices can consume greater than 1 A of current.
Fortunately, the power domains that tend to have the highest current consumption also have the lowest voltage requirements. For example, it’s common now to see devices that use sub-100-nm lithography nodes and have a core operating voltage of less than 1.2 V. However, for legacy purposes, power supplies for I/O domains and embedded converters require operation in the 3.3-V to 6-V range with only 10% to 20% of the current required by the core.
These factors need an optimal ATE device power supply to provide variable power compliance as well as variable channel density. Accordingly, power- supply channels designed to supply 3 W of power need to provide 3 A at 1 V, 910 mA at 3.3 V, and 600 mA at 5 V. When these requirements are combined with the need for multiple supplies per device, an ATE infrastructure that can support 100 3-W power supplies must have the flexibility to support 300 1-W power supplies to cover the wide variety of test insertions (Figure 1).
Figure 1 Variable Power Supply Compliance for a Fixed 3-W Channel
Voltage Excursion Detection and Management
Prior to releasing an automotive microcontroller device test program to production, it is necessary to ensure that the test program does not exhibit excessive voltage excursions. These also may be referred to as glitches or spikes.
The occurrence of voltage excursions is most commonly associated with a sequence of instrument transitions and connections. Voltage excursions on a device power supply can occur under different dynamic and control-loop scenarios.
Verifying that automotive microcontroller test programs do not exhibit voltage excursions is a requirement to ensure quality. It can be a very time-consuming task. It is not uncommon to see large device test programs require two to four weeks of checkout (including I/O and analog pins, not just device power pins).
Many times, the causes for glitches and voltage excursions on device power supply pins differ from other device I/O pins. Microcontroller device I/O pins often are multipurpose pins having both digital and analog functionality depending on the device mode. Testing these multipurpose pins usually involves switching between different ATE instruments throughout the test flow. Hot switching between these instruments is a common cause for glitches. By comparison, excessive voltage on power pins tends to occur for different reasons and can be dependent on the power-supply architecture.
One common cause for a voltage excursion is a change to the power supply’s current meter range. Multiple meter ranges are required to address the dynamic range of the measurements themselves, which often cover six orders of magnitude. While there are various architectures for current metering, the most common is either a serial or parallel shunt configuration (Figure 2).
Figure 2 Example of a Power-Supply Current Meter Shunt Circuit Implementation
An ideal serial shunt configuration ensures that nothing is switched out of the shunt path when the current meter range changes are made. However, when compromises are made in the shunt implementation, a meter range change can result in a change of impedance from the power supply to the DUT. When this impedance change occurs, the power supply’s control loop needs to drive the voltage at the DUT back under control.
Another cause of voltage excursions is a change to the device’s dynamic load while at a fixed voltage. If a device increases the amount of power-supply current draw, it can temporarily lower the supply voltage, causing it to droop. Likewise, if the power-supply current draw rapidly decreases, the power-supply voltage level can temporarily increase or kick.
The magnitude and duration of these voltage excursions depend on the power supply’s control-loop architecture and interaction with load-board components and parasitic effects. Modern devices are becoming more sensitive to the effects of droop and kick because of lower power-supply voltages. Many devices tend to have a 15% to 20% tolerance for voltage excursions relative to their nominal operating point. So where a 200-mV excursion is only a 4% impact on a 5-V device, it is a 20% impact on a 1-V device. This situation can be further exacerbated by devices that have power-supply monitoring circuits where an overvoltage or undervoltage circuit can trigger and cause the device to switch operating modes.
Ideally, the ATE power supply would not exhibit voltage excursions, but in practice, most do. Accordingly, having the ATE instrument capable of detecting and reporting these events is very useful.
A simple voltage excursion detection circuit is shown in Figure 3. The allowed high and low excursions are programmed as inputs to the comparator circuits (labeled POS and NEG), and the other inputs monitor the power supply output. If the power-supply output voltage exceeds the programmed excursion limits, the comparator state toggles and is latched. ATE system software can read back this latched state and report the issue to the user. With this information, the user can dig into the details of the test that experienced the problem to identify the root cause and modify it.
Figure 3 Glitch Detect Circuit Example
Another useful debug feature is current and voltage profiling. More recent ATE instruments provide integrated hardware and software features, allowing the instrument to digitize its own voltage and current waveforms. The digitized waveforms can be displayed by the system’s plotting utilities, eliminating the need for an oscilloscope.
These debug features are examples of ways in which ATE instruments can assist with the steps required to ensure that the voltage excursions are within tolerable limits. These types of tools, if available during a device’s release to production, improve engineering efficiency as well as time to market.
Conclusion
While often viewed as the simple instrument in an ATE system, the device power supply is experiencing ever-increasing technical challenges and addressing those challenges by being more flexible and feature rich. With these improvements in power-supply architecture, semiconductor manufacturers can test the devices of today and tomorrow to achieve the cost of test and quality of test needed for the wide variety of microcontroller devices.
About the Author
Jeremy Campbell is the manager of the Consumer Business Unit’s Factory Applications Engineering team in the Semiconductor Test Division of Teradyne and has more than 10 years of experience in the ATE industry. He received a B.S.E.E. from the University of Windsor. Teradyne Semiconductor Test Division, MS 600-2, 600 Riverpark Dr., North Reading, MA 01864, 978-370-1074, [email protected]