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TMS320LF2407A in mixed voltage system design

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With portable digital electronic products, digital mobile phones, hand-held test instruments such as the rapid development, require the use of small size, low power consumption, battery power small devices, so that the operating voltage of integrated circuits has been reduced to 3 from the 5V .3 V or even lower, such as 2.5V and 1.8V. However, there are still many 5V power supply and digital logic devices can be used, so there will be many of the design of 3.3V and 5V logic device logic devices co-exist, and different power supply voltage in the same circuit board in the mix. With the introduction of a lower standard voltage, mixed-voltage system will replace the single-voltage systems, and will exist for a long time.

1 TMS320LF2407A Overview

TI Introduces TMS320LF2407A is of a fixed-point dsp controller, which uses high-performance static CMOS technology, allows the supply voltage reduced to 3.3V, the controller reduces power consumption; 40MIPS makes the implementation of the speed instruction cycle shortened to 25ns (40MHz), thus enhancing the ability of real-time control of the controller; integrates 32K words of flash memory (which can be encrypted), 2.5K of RAM, 500ns conversion time of A / D converter chip provides event management can be PWM motor to meet a variety of interface and I / O functions, in addition to the provision applicable to the field of industrial control some special features such as watchdog circuit, SPI, SCI, and CAN controller and so on, so that it can be widely used in industrial control.

TMS320LF2407A in mixed voltage system design

However, in practical applications, but also the need for external TMS320LF2407A the necessary expansion of procedures such as data areas and the expansion of the district, CAN drivers and so on, to meet the needs of the entire application system, the actual needs.

2 power supply design

TMS320LF2407A the operating voltage is 3.3V, and many peripheral devices commonly used power of the main tasks usually 5V, therefore TMS320LF2407A posed as the core of the application system must be a mixed-voltage systems. With the complete system, compared to 3.3V mixed voltage systems because of the lack of low-voltage devices, there is apparently a lot to be desired. One of the main drawbacks is the requirement of multi-power supply, a typical system requires 3V, 5V, +12 V/-12V, or even higher voltage. Design of a target is to reduce the number of power supply requirements and to reduce supply voltage requirements of these the number of devices. In order to reduce the power required for more than the number of additional devices, many manufacturers have provided a wide range of voltage chips. At the same time, as technology advances, there will be a more low-voltage devices, and then to gradually eliminate the requirement for more power and cost of these power and complexity. TMS320LF2407A applications for the purposes of, first of all to solve the power problem TMS320LF2407A. 3.3V power supply usually have to solve the following program.

Pressure resistance of 2.1

Using the method of pressure resistance is relatively simple, the principle as shown in Figure 1. However, the actual output voltage of the circuit is clearly less than 3.3V, and as the load changes, the output voltage will also fluctuate. In addition, this circuit is also relatively large power consumption. However, its relatively low cost and simple structure, can serve as an emergency response program. For low-power systems and high power requirements of the system, not suitable for the use of such programs.

2.2 Direct use of power modules

Switching power supply, taking into account the complexity of design, some companies such as Agere (Lucent's Microelectronics Department of the original), Ericsson, Vicor, such as the introduction of a switching power supply technology based on low-voltage output power supply module. The modules have a high reliability and efficiency, electromagnetic radiation of small, and many modules can also be isolated to achieve power. These easy-to-use power supply module, only very few external components to increase, but prices are rather expensive.

TMS320LF2407A in mixed voltage system design

2.3 the use of power conversion chip linear regulator

Linear regulator chip is one of the most simple power conversion chip is basically no need for external components. However, the traditional linear regulator, such as the LM317, the input voltage requirements of high output voltage than 2V or more, or else can not work properly. For 5V input and output can not be reached 3.3V. Faced with the demand for low-power, many power chips has introduced low-voltage linear regulators (LDO). This pressure only power chip 1.3V-0.2V, can achieve 5V to 3.3V/2.5V, or 3.3V to 2.5V/1.8V requirements. LDO required less the number of peripheral devices, easy to use, low cost, small ripple, no electromagnetic interference. For example, TI's TI company TPS73xx series is designed to tie in with the DSP chip for power conversion, the output current can reach 500mA, and the interface circuit is very simple, just connect the external resistor is necessary, we can achieve the power conversion. The series is divided into a fixed output voltage of the chip and the chip adjustable voltage output. However, the efficiency of the chip is usually not very high, and relatively large power consumption.

Power supply design program which depends on the specific requirements of the system. Typically, low-power or low power supply efficiency requirements, they can adopt LDO. But for high-power or higher power supply efficiency when the power supply module should be used. One of the features of TMS320LF2407A is low-voltage work, and its power consumption is relatively low, so the use of TI's more appropriate to TPS73xx series. Which, TPS7333 is a fixed 3.3V output voltage of the power conversion chips, the power supply is suitable for the needs of TMS320LF2407A.

3 logical interface design

TMS320LF2407A as a result of the introduction of the logic of different voltage systems will coexist in the same circuit board in, for example, in the same circuit board in the existence of two kinds of 3.3V and 5V logic system. Therefore, in the design of the interface between logic devices, the use of appropriate methods, to avoid the logic of different voltage device interface problems, designed to ensure the reliability of data transmissions circuits.

3.1 At the same time, logic-level interface is not the problem

In mixed voltage systems, different supply voltage interface logic of each device when the following three main issues: add input or output pin on the maximum allowable limits of voltage; power between the two current issues of each series; must input to meet the threshold issue of conversion.

TMS320LF2407A in mixed voltage system design

Add devices to the input pin or output pin voltage is usually limited. These pin-diode or the receipt of digital components Vcc. If access to high voltage, current will be. Through the diode or the flow of digital power components. For example, 3.3V termination devices on the 5V input signal, then 5V to 3.3V power supply will be charged the current will be sustained damage to diodes and circuit components.

Waiting or when the power-down mode, 3.3V power supply voltage down to 0V, current will flow to ground, which has been high on the bus drop-down to ground, which will lead to data loss and damage components. It should be noted that: whether in the work of the state of 3.3V or 0V to wait for the state, current flows are not allowed to Vcc.

In addition, the device driver with 5V devices 3.3V will be a variety of different situations, and TTL and CMOS level conversion between the different situations there. In these cases, the drive must meet the receiver input level conversion, and tolerance must be sufficient to ensure that components do not damage the circuit.

3.2 3.3V and 5V logic interface between the

TMS320LF2407A typical operating voltage is 3.3V, the I / O port level is 3.3V. In the design of peripheral interfaces, the external device if the voltage is 3.3V, the interface circuit on the relatively simple, can be directly connected. CYPRESS, such as the 64Kxl6 the CY7C1021BV33 is a high-performance CMOS static RAM, can be linked directly with the TMS320LF2407A of TMS320LF2407A expand data areas.

However, because there are many common external voltage chips are 5V, such as EPROM, etc., and TMS320LF2407A the I / 0 voltage is 3.3V, I / O level is 3.3V, and 5V in the TMS320LF2407A between the external chip interface there is the issue of reliability. Figure 2 to 5V CMOS, 5V TTL and 3.3V TTL electronic standards conversion. Which, VOH that the minimum output high voltage, VIH high that the minimum input voltage, VIL that the maximum input low voltage, VOL that the maximum low-level voltage output. As can be seen from Figure 5V m and 3.3V TYL standards conversion is the same, and 5V CMOS Xiaoping conversion is different. As a result, the system will be 3.3V and 5V interface will not take into account the difference between the two.

Therefore, the design of 3.3V and 5V logic interface between the device should consider the following four cases:

(1) 5V TTL devices are 3.3V TTL device driver (LVC). 5VTTL and 3.3V TTL as a result of e-standards are the same, so if the device can be 3.3V TTL voltage 5V bear, between the two devices can be directly linked to gold, without the need for additional devices. However, if the device can not afford 3.3VTTL voltage 5V, you need to add a dedicated circuit or device-level converter, such as in the interface design, add a diode to generate an additional voltage drop of 0.7V. Of course, the best approach is to increase between the two devices of a standard TI's CBT buffer, the buffer in the integration of the diode.

(2) 5V CMOS devices are 3.3V TTL device driver (LVC). It is clear that the two-level conversion is not the same. 5V CMOS for the VOH and VOL, and the 3.3V TrL ten VIH and VIL analysis can be drawn, although there existed a certain degree of difference, but can withstand voltage of 5V and 3.3V devices interface 5V CMOS devices, they can work properly. That is to say, 5V CMOS devices can drive those who can afford the 3.3V voltage 5V devices.

(3) 3.3V TTL Device (LVC) drive 5V TTL devices. As a result of the conversion between the two standards is the same level, so when the two connected, no additional device. 5V TTL devices as VIH and VIL levels are 2V and 0.8V, so long as the 3.3V device VOH and VOL levels are 2.4V and 0.4V, 5V TTL devices can be input to identify for the effective level.

(4) 3.3V TTL Device (LVC) drive 5V CMOS devices. Conversion between the two standards are different. As can be seen from the chart, 3.3V device VOH for the 2.4V, the 5V CMOS and VIH to 3.5V. Even if the voltage output 3.3VLVC reached 3.3V, not 5V CMOS to meet the high minimum required, the 3.3V TTL Device (LVC) is not directly driven by the 5V CMOS devices. In this case, you can use the TI to provide a drive, such as SN74ALVCl64245 and SN74ALVC245. Such a dual voltage power supply chips, while power supply is 3.3V, while the other side of the 5V power supply, it can effectively solve the 3.3V and 5V CMOS devices between device-level conversion.

3.3 TMS320LF2407A and peripheral interface devices

TMS320LF2407A in the design of peripheral interfaces, the first careful analysis of needs and related peripheral devices TMS320LF2407A level standards conversion, which can be from the device to obtain electrical parameters table. TMS320LF2407A, M27C516 (EPROM) and the level of standard 80C250.

TMS320LF2407A in mixed voltage system design

M27C516 is a 32K × l6 the EPROM, the device can be used to carry out the procedure TMS320LF2407A expansion. TMS320LF2407A the VOH and VOL, respectively, for 2.4V and 0.4V, and VIH and VIL of the M27C516 are 2.0V and 0.8V, the M27C516 from TMS320LF2407A single control line and address lines can be directly linked. But can not afford LF2407A voltage of 5V, the M27C516 to TMS320LF2407A from the data lines can not be directly linked. The solution is to add a buffer in the middle pieces, such as 74ALVCl64245. It uses dual-voltage power supply, while using 3.3V power supply, the other side of the use of 5V power supply, so the power can be 3.3V to 5V level shifters level, on the contrary can also convert 5V level 3. 3V level, it can be used as two 8-bit bus driver or a bus driver 16. TMS320LF2407A and 74LVCl64245 interface through M27C516 diagram shown in figure 3.

Bus interface buffer can be used to increase the way pieces, but the serial interface, there is no need to increase the buffer cases, to design a simple circuit to achieve, such as the 82C250 interface. 82C250 is the drive CAN bus controller and physical interface between the offer of bus differential transmit and receive functions. TMS320LF2407A of VOH is 2.4V, and VIH of 82C250 is more than 3.5V, it is clear that not TMS320LF2407A driver 82C250; At the same time, 82C250 than VOH of 4V, the VIH and TMS320LF2407A maximum 3.6V, the voltage of 5V can not afford, Therefore, with the 82C250 interface TMS320LF2407A need an additional level-shifting circuit. Figure 4 is a composed of resistance and diode level-shifting circuit, in CANTX output, an increase of a diode, so that the TXD voltage received increased 0.7V; while RXD level of resistance after two sub - pressure, making CANRX level can be received to ensure that within 3.3V.

Of course, in between CANTX and TXD can also use to achieve 74LVC07 interface. This is a simple level shift device, which uses an open-drain buffer to drive 5V CMOS input devices. As a result, between CANTX and TXD add a 74LVC07, and in its output can be received through the pull-up resistor on the 5V power supply, thereby driving TXD.

5V and 3.3V low voltage devices or even devices co-exist in a system, this situation already exists and will exist for a long time. Therefore in the design of this mixed-voltage systems, which require careful analysis of the interface logic device. TMS320LF2407A For example, it is a low-voltage chips, if the interface with other chip designs were not good, not only not be able to demonstrate the characteristics of its low power consumption and will reduce the reliability of data transmission, and may even damage the chip. This article described several ways, by the experiments with high reliability.

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