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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Decoder logic analyzer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer How to understand the operation of a device in which FPGAs or custom VLSIs are used, without having a detailed description of it? Only by analyzing the signals at the inputs and outputs of microcircuits and connector pins. The proposed device can help with this. In some cases, it will successfully replace a multi-channel digital storage oscilloscope. With the help of the analyzer, the author of the article managed to repair several video game consoles. The processor of a typical computing system has access to each of the memory chips and to all input/output ports. Turning to them, it exposes a certain combination of logical levels on the address and control buses. The device selection signal (memory chip or I/O register) forms an address decoder (DA), which generally performs a logical AND operation on the direct and inverse values of the processor signals. In modern equipment, DAs are often placed inside FPGAs and custom microcircuits with operation logic unknown to the user. Failed devices with such microcircuits can sometimes be repaired by replacing the failed built-in DA with a self-made external one assembled from available parts. But for this, first of all, it is necessary to determine which signals of the processor system are fed to the inputs of the DA. Having a serviceable device similar to the one under repair, you can use a multi-channel digital storage oscilloscope to remove and carefully analyze the timing diagrams of numerous signals. However, this will take a lot of time and patience. In some cases, it is easier to use the decoder logic analyzer (hereinafter referred to as the analyzer), the diagram of which is shown in Fig. 1. By applying an output signal YES to its input "CS" and connecting the input "ADR" in turn to various circuits of the device under test, it is possible to quickly find the signals involved in the operation of the decoder and determine their polarity. The analysis is based on the fact that the signal applied to the input "ADR" with a high probability belongs to the number of input YES, if its logic level is the same at the beginning of each pulse at the input "CS" and remains unchanged during the entire pulse.
Traditionally, in most microprocessor systems, the active level at the YES output is low. But exceptions are possible. Switch SA1 allows you to select as active a high or low signal level at the "CS" input. Depending on its position, the element DD1.3 inverts or does not invert the signal. Before comparing the signal levels at the inputs "CS" and "ADR" elements DD3.1, DD3.2 and DD1.4 delay the latter by several tens of nanoseconds. This compensates for the delay in the analyzed DA and in the DD1.3 element. The comparison itself is performed by the elements DD3.3 and DD3.4, the pulses at the outputs of which appear only if the input signals do not coincide in time. Circuits R5C3 and R6C4 suppress short-term emissions (so-called "needles") caused by transients. Two RS-flip-flops are assembled from the elements of the DD5 chip. One of the inputs of each receives pulses from the corresponding comparison node, the other - from the reset pulse generator on the elements DD1.1, DD1.2. Periodic reset of triggers allows you to monitor the dynamics of the process under study. Reset pulse duty cycle - 500... 1000, repetition period - 80... 120 ms. Thanks to the use of the DD1 chip of the KR1533 series, the value of the resistor R3 was chosen quite large (by TTL standards), which made it possible to reduce the capacitance of the capacitor C1. The DD4 counter serves as a signal change detector at the "ADR" input. If between two reset pulses from the output of the element DD3.1 at least two pulses come to input 5 DD4, the high level established at the output 2 of the counter will go to the inputs of the elements DD2.3 and DD3.4, allowing the status of the triggers to be indicated by the LEDs HL1, HL2 before the arrival the next reset pulse to the input R of the counter. Simultaneous illumination of the LEDs means that the signal applied to the "ADR" input does not participate in the operation of the analyzed DA. If only one of the LEDs is lit (sometimes with a "wink"), the signal level at the "CS" input is active when the signal level at the "ADR" input is low (HL1 is on) or high (HL2 is on). With a constant logic level of the signal at the input "ADR" (for example, when this input is not connected anywhere), the state of the counter DD4 remains zero and the indicators are extinguished. Practice has shown that such blocking significantly reduces the probability of false analyzer readings. Low-resistance resistors R1 and R2 are connected in series to the input circuits of the analyzer. They are necessary to eliminate the "ringing" on the differences of the analyzed signals, which occurs with long connecting wires. If protection of inputs from high positive and negative voltages is required, diodes VD3-VD6 are installed in the analyzer, shown in the diagram (Fig. 1) by dashed lines. However, the inherent capacitance of the diodes degrades the performance of the device. Diodes can be from the KD521, KD509 series or similar imported ones. The analyzer is powered from any 5 V voltage source, including the one available in the device under test. The consumed current does not exceed 35 mA. Schottky diode VD1 protects against reverse polarity connection to the source. If this is not necessary, the diode can be eliminated by replacing it with a jumper. To obtain a high logic level voltage applied to some inputs of logic elements and microcircuits, the DD2.1 element was used. As HL1 and HL2, LEDs of any type and color of glow are suitable, although a red-green pair looks better. Chips DD1 and DD3, it is desirable to use the KP1533 series. The rest can be from different TTL series, for example, K555, K155. Having applied to the "CS" input of the assembled analyzer any pulses of TTL levels with a frequency from hundreds of hertz to a few megahertz, make sure that when it is not connected anywhere or connected to the +5 circuit, the LEDs HL1, HL2 are off at the "ADR" input. After connecting the "ADR" input to the common wire, the LEDs flash briefly and go out. If you apply the same pulses to the "ADR" input as to "CS" (by connecting the inputs), when the SA1 switch is closed, only the HL1 LED should light up, and when the switch is open, only HL2. An example of the practical application of the analyzer is the study of the cartridge selection signal generation unit in the Sega video game console (see Ryumik S. Features of the circuitry of 16-bit video consoles. - Radio, 1998, No. 4, 5, 7, 8). The "CS" input is connected to one of the ROM selection circuits - contacts B16 (OE) or B17 (CS) of the "CARTRIDGE" connector of a working set-top box. Install and launch any game cartridge. With the probe connected to the "ADR" input, touch each pin of the "CARTRIDGE" connector in turn and for some time observe the state of the analyzer's LEDs. In doubtful cases, press the "RESET" button of the game console. In this way, contacts are found, when connected to which both LEDs light up in one position of the SA1 switch, and only one of them lights up in the other. Sometimes, to make sure the analysis is correct, you have to repeat it with a different cartridge. Of course, there is no guarantee that all the necessary signals will be found. It cannot be ruled out that some of them are "hidden" very deep inside the VLSI and are physically inaccessible. And still... The experiment showed that cartridge selection pulses CS coincide in time with high levels of signals A21 and A22, and OE - with low levels of WE1 and WE2. As a result, it was possible to manufacture a node on just one microcircuit, replacing faulty decoders. Its scheme is shown in Fig. 2, the crosses on it mark the video set-top box circuits that must be broken when installing the node by cutting the printed conductors. Naturally, in the event of a malfunction only in the OE signal conditioning circuit, there is no need to redo the CS circuit, and vice versa.
With the help of this unit, it was possible to repair several "hopeless" copies of "Sega" models NAA-2502 and MK-1631-07 with defects in the VLSI video processor U3 (TA-06) and multiprocessor U4 (with the inscription "97xx" or "98xx"). An external symptom of a malfunction was the complete absence of image and sound, pulses of access to the CS and (or) OE cartridge, a high logic level at pin B31 (CHECK) of the "CARTRIDGE" connector. Author: S.Ryumik, Chernihiv, Ukraine
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