Electronics Production | January 31, 2011
Understanding ISP Wiring
With the high bit rates of today’s In-System Programming protocols, careful wiring between the programmer and the programming/testing fixture must be considered.
Fewer Wires, Higher Speeds In-System Programming (ISP) is becoming the programming solution preferred by small and big OEM/EMS alike. Since each silicon manufacturer implements its own programming protocol(s), every device family has its own particular programming interface. Nonetheless, the lines needed to program a typical device have decreased during the years. Modern devices only need very few lines to be programmed; in some cases, just one line is needed (Fig. 1). At the same time, the devices’ non-volatile memory is rapidly increasing (Fig. 2). To keep programming times low, the ISP lines must therefore be capable of sustaining higher communication bit rates. Higher bit rates mean that the wiring of the ISP lines inside the programming/testing fixture has to be carefully thought. Fixture Wiring Usually, an Automatic Test Equipment (ATE) performs parametric and functional tests on the Unit Under Test (UUT) that is placed inside a custom, unit-specific test fixture. The text fixture routes several ATE control lines to the various test points on the UUT. The same fixture is used to in-system program the target device(s) (or DUP, Device Under Programming) in the UUT that needs to be programmed. In-system programming usually takes place after the component parametric test and before the functional test. Multi-PCB panels add to the complexity of the wiring inside the fixture; fixtures with hundreds of nails are not uncommon (Fig. 3). While the vast majority of connections required for board testing are low-speed transmission lines, the wiring of the lines dedicated to programming must take into account the high speed of modern ISP. ISP Wiring Issues One of the most important parameters to take into account for ISP wiring is connection length. Usually (but this is not always known to the test engineer), ISP signal integrity starts to degrade with connections longer than 40-50cm. For this reason the programmer should be placed as near as possible to the fixture, better yet inside the fixture. The 50cm limit should not be taken as an absolute maximum: it depends on the ISP protocol. Asynchronous protocols such as UART, for example, allow for longer connections; Freescale’s BDM protocol, due to the high bit rate and driving characteristics, requires shorter wiring. An important role in an electric communication is played by the transmission line. Due to the high number of wires and to their connection typology (usually wire wrap, making solderless connections of wires to terminals), fixture wiring is typically done with single-core, isolated wires with AWG from 20 to 30. Since almost all programmable devices (serial memories, microcontrollers, etc.) feature single-ended, CMOS lines, almost all In-System Programmers in the market feature single-ended, CMOS driving of ISP lines (Fig. 4). This transmission line is easier to implement than a differential line, but it offers less immunity to common- mode noise. Programmer and DUP usually share the same ground potential and, with the simultaneous activity of several digital lines, electric noise will add both on the transmitter and receiver end—possibly corrupting the transmission. Fig. 5 shows a signal affected by common-mode noise. Another source of noise can be the crosstalk phenomenon. Crosstalk arises when a signal affects another nearby signal. Usually the coupling is capacitive, and to the nearest neighbor, but other forms of coupling and effects on signal further away are sometimes important. Fig. 6 shows an example of crosstalk between two conductors inside a fixture. The CH1 signal is affected by the CH2 signal. The steeper the edges of the CH2 signal, and the higher the CH1 line impedance, the higher the crosstalk. ISP protocols such as I2C, ICC, etc., where the high logic state is obtained through open-drain driving, are more subject to the crosstalk phenomenon. In order to reduce crosstalk, twisted pair cabling should be used, where a “shielding” conductor (only one end of which is connected to ground, usually on the programmer side) is twisted together with the main conductor (alternatively, a coaxial cable can be used). Fig. 7 shows the benefits of such a cabling. Fig. 8 shows how cable twisting should be implemented. High-Speed Signals Crosstalk effects are empathized by modern In-System Programmers since, in order to reach the higher bit rates of today’s ISP protocols, they employ high-speed single ended line driving. To help avoid crosstalk, the fixture should have a ground plane on the plate hosting the nails (Fig. 9). Additionally, ground connections should be redundant; all the ground pins of the programmer should be individually connected to ground nails on the fixture, and these should be connected to each other, as shown in Fig. 10. Impedance Matching Impedance matching is another factor that improves signal transmission. A transmission line is said to be “matched” when its characteristic impedance (Zo) is equal to that of the transmitter (ZoTX) and to that of the receiver (ZoRX). When the transmission line is matched, signal reflection and interferences are null (Fig. 11). Perfect impedance matching is impossible to obtain in real-world conditions. In particular, the output impedance of a programmer line (ZoTX) is typically in the order of hundreds of Ohm, while the input impedance of a microcontroller line (ZoRX) is usually in the order of hundred thousands of Ohm. Additional circuitry on the UUT tied to the transmission line may make things worse. As an example of unmatched transmission line, see Fig. 12. A strong undershoot is present on the signal. A pull-down resistor between signal and ground reduces this phenomenon. The resistor, whose value of 390 Ohm has been empirically found, has been placed between the contacting nail and the fixture’s ground plane. Fig. 13 shows the effect of the pull-down resistor. The pull-down resistor, however, draws a significant amount of current from the transmitter. For a 5V CMOS signal, the current drawn is I = 5V / 390Ohm = 13mA, which is comparable with the maximum source current of a CMOS line. It is suggested to use non-inductive resistors, such as carbon resistors. Eye Diagram An eye diagram allows to evaluate the quality of a transmission line. An eye diagram is an oscilloscope display in which a digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to crosstalk interference and noise appears as closure of the eye pattern. Fig. 14 shows the eye diagram captured on the MOSI signal of a SPI protocol, one of the various protocols used in In-System Programming. If the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too noisy, or too slow to change, or have too much undershoot or overshoot, this can be observed from the eye diagram. While the analysis of the eye diagram is suggested for advanced signal troubleshooting, it is by no means mandatory. Traditional scope captures of the signals involved in ISP are enough to evaluate the quality of the fixture connections. ----- Author: Stefano Querin; R&D Department; Algocraft
Big Ass Fans adds third Hentec/RPS machine Complementing the purchase of two Hentec Industries/RPS Automation Vector 300 selective soldering machines in 2018, Big Ass Fans (BAF), a high-volume, low- speed (HVLS) airflow manufacturer, has just added a third machine to its factory in Newman Lake, Washington.
CE3 Electronics Inc. taps MIRTEC for 3D AOI technology Canada-based CE3 Electronics has purchased a MIRTEC MV-6 OMNI 3D AOI machine.
Gémosz is still looking to grow – but the pandemic brought new perspectives Winston Churchill famously said, “Never let a good crisis go to waste”. While this was said in a vastly different context than the current crisis of the COVID-19 pandemic, there is still truth to the fact that a crisis not only allows for change – but sometimes forces it.
North American PCB industry sales up 4.3% in April The IPC says that total North American PCB shipments in April 2020 were up 4.3% compared to the same month last year. Compared to the preceding month, April shipments fell 18.2%.
Volkswagen intensifies e-mobility activities in China Volkswagen has initiated the next chapter for its business in China. The automotive giant plans to increase its share in JAC Volkswagen, its joint venture for e-mobility.
GPV Selects Aegis as global smart factory partner Driven originally by accelerating customer expectations for traceability, GPV has selected Aegis’ FactoryLogix Smart digital manufacturing solution, targeting multiple values and benefits.
REDCOM EMS expands certification for medical device manufacturing New York-based REDCOM EMS has secured ISO 13485:2016 certification, complementing the company’s existing ISO 9001:2015 and AS9100D certifications.
Ozark Integrated Circuits nabs U.S. Air Force grant Arkansas-based Ozark Integrated Circuits Inc., located in the Arkansas Research and Technology Park at the University of Arkansas, has received a USD 750,000 award from the U.S. Air Force.
Norwegian EMS adds employees to Polish unit The last couple of months Norautron has welcomed three new colleagues at its office in Sdunska Wola, Poland.
Daimler’s battery production in Kamenz gradually increases production Local battery production is described as an important success factor for Mercedes-Benz AG’s electric offensive and a key element to flexibly and efficiently meet the global demand for electrified vehicles.
Tepcomp invests in new additional production line Finnish EMS provider, Tepcomp Group, has made a significant investment at its production plant in Turku. The new surface mount technology production line will significantly improve the productivity and energy efficiency of the plant.
Valoe is on schedule with both production and expected deliveries Valoe says it will start manufacturing IBC test modules in Juva, Finland in June and cell deliveries from Lithuania to customers is scheduled to start according to plan.
AMTE Power and Britishvolt sign MoU for GigaFactory AMTE Power and Britishvolt plans to investigate collaborating to build the UK’s first full cycle battery cell GigaPlant, servicing the automotive and energy storage markets.
ALLPCB’s new SMT factory has launched production On May 10 2020, ALLPCB’s new – self-operated – SMT factory in Guangde, Anhui Province, China was officially put into production.
Schweizer starts production at its new location in China Following a construction period of one and a half years, production has now started at the company’s new high-tech printed circuit board plant in Jiangsu, China.
Jenoptik supplies generators to a US customer Through its Vincorion investment, Jenoptik received a long-term order for generators that an unnamed US customer is integrating into military vehicles.
Rolls-Royce takes heavy hit from COVID-19 – reduces workforce by 9000 The impact of COVID-19 on Rolls-Royce and the whole of the aviation industry is unprecedented. RR has already taken action to strengthen the financial resilience of its business and to reduce cash expenditure in 2020. However, this will not be enough.
Data Link Solutions to provide MIDS cabinet terminals to US Navy The U.S. Navy has awarded a USD 3.2 million production contract to Data Link Solutions (DLS), a joint venture between BAE Systems and Collins Aerospace, for new Multifunctional Information Distribution System (MIDS) cabinet terminals.
Incap restarts production in India In a move to contain the COVID-19 pandemic, the government of India instructed all state governments to introduce a lockdown in March. As a consequence, Incap had to close its factory in Karnataka, India on 23 March.
NAND Flash Revenue Undergoes 8.3% QoQ Growth in 1Q20 NAND Flash bit shipment in 1Q20 was relatively on par with 4Q19, says the DRAMeXchange research division of TrendForce. The overall ASP of NAND Flash products also climbed during the period. As a result, the global NAND Flash revenue for the quarter went up by 8.3% QoQ to USD 13.6 billion.
Gorenje reduces the number of layoffs At the end of April, due to a sharp drop in orders and revenues as a result of the COVID-19 pandemic, the Hisense Europe Group launched a redundancy program in four companies in Slovenia.
NA semiconductor equipment industry posts April 2020 billings North America-based manufacturers of semiconductor equipment posted USD 2.26 billion in billings worldwide in April 2020 (three-month average basis), according to SEMI.
AWS Electronics to wear the Incap name Following the acquisition of AWS Electronics Group by Incap Corporation in January 2020, the company's units in the UK and Slovakia will now be trading as Incap Electronics UK Ltd. and Incap Electronics Slovakia s.r.o.Load more news
- Rolls-Royce takes heavy hit from COVID-19 – reduces workforce by 9000
- COVID-19 highlights the gravity of the secondary market for SME
- China falls far short of its "Made-in-China 2025" goal
- Data Link Solutions to provide MIDS cabinet terminals to US Navy
- GlobalFoundries to implement ITAR at US manufacturing facility