Application Note Power-Up and Power-Down Behavior of 54SX and RT54SX Devices I n tro du ct i on One of the key benefits of Actel's nonvolatile antifuse FPGA technology is the ability of the devices to be live at power-up. Since no configuration PROMs are required to download design information to the device, Actel FPGAs are ready to run as soon as the power-up sequence is complete. However, there are a few restrictions that need to be considered while designing with these devices. This Application Note explains all the requirements of Actel's 54SX and RT54SX devices during power-up and power-down. In addition, this Application Note also discusses the behavior of outputs during power-up, explains when the device is functional, and provides recommended ramp rates for power-up. This report was measured in a laboratory environment. The values reported are typical values, and therefore, not guaranteed maximum or minimum values. operation mode. The flip-flops in the design are not initialized to a known state during power-up. Thus, the user needs to provide an external signal to reset the flip-flops upon power-up. P ower Su pply R equ ir em ent s Table 1 lists the specific power supply requirements for the 54SX and RT54SX FPGAs. Table 1 * Power Supply All 54SX and RT54SX 54SX16P Bac kgr ound In order to fully understand the power-up characteristics of Actel 54SX and RT54SX FPGAs, the user must first learn some background information about the power-up circuitry and understand the function of each power supply. The three power supplies available on 54SX and RT54SX devices are: 1. VCCI: This is the power supply for all I/Os on the device 2. VCCR: This is the reference voltage used for input tolerance 3. VCCA: This is the power supply for the internal array During power-up, the VCCA voltage is used to initiate a sequence of events that puts the device into functional mode. Once VCCA reaches approximately 2.0V (See "I/O Behavior" for minimum values and their dependency on ramp rate), a sensing circuit enables an internal charge pump. The charge pump provides a high voltage (5-8V) that is used to turn on isolation devices throughout the FPGA. These isolation devices reside on the input and output of every logic module in the chip and are used to isolate the logic modules from high voltages during programming. During normal operation, all isolation transistors are turned on, which allows the logic array and I/Os to function according to the design. The VCCA-controlled sensing circuit also places the JTAG circuitry in TEST-LOGIC-RESET mode, so the device is in normal J an u a r y 2 0 0 1 (c) 2000 Actel Corporation VCCA VCCI VCCR Input Tolerance Output Drive 3.3V 3.3V 5.0V 5.0V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 5.0V 5.0V 3.3V 3.3V 5.0V 5.0V 5.0V 5.0V Po w e r - U p B e ha vi o r I / O B e h a v io r During power-up, a pull-up transistor between each I/O and VCCI will be turned on. Therefore, the I/Os will appear to drive LOW until the device is active. The amount of time the I/Os drive LOW depends on the ramp rates of the board's power supplies and the time delta between VCCR and VCCA. After the I/Os drive LOW, they will behave according to the design. Table 2 on page 2 summarizes the VCCA voltage at which the I/Os behave according to the user's design for all 54SX and RT54SX devices at room temperature. Table 2 on page 2 assumes a linear ramp-up profile to 3.3V. The reset process that enables the outputs begins when VCCA reaches the nominal value of about 2.0V (+/- 0.5V). Table 2 on page 2 characterizes the voltage at which the outputs become active against the VCCA value for the listed power-up rate. For example, if the user powers-up an 54SX32 device at 0.66V/ms, the outputs become active when VCCA reaches about 2.1V. The point at which the I/Os become enabled depends on the power-up rate and process variations between different devices. Note, results may vary from those presented here due to these dependencies. 1 P o w e r - Up a n d Po w e r -D o w n B e h a v io r o f 5 4 SX a n d R T5 4 SX D e v ic e s Transient Current When VCCA reaches a voltage of about 2.0V, the VCCA power supply will experience a transient current for approximately 500ns. This current is due to the sudden switching of all the internal logic modules when the device initialization is applied. The value of the transient current is highly dependent on the VCCA ramp-up rate and the size of the device. Table 3 summarizes typical values of the transient current for different ramp rates and device sizes at room temperature. Table 2 * VCCA Voltage where I/Os Become Active (V) 0.33V/s 0.033V/s 6.6V/ms 3.3V/ms 0.66V/ms 0.33V/ms 0.132V/ms 0.033V/ms 54SX08 3.3 3.3 2.6 2.2 2.0 2.0 1.9 1.9 54SX16 3.3 3.3 2.9 2.2 1.8 1.7 1.7 1.5 54SX16P 3.3 3.3 2.9 2.5 2.0 1.9 1.8 1.7 54SX32 3.3 3.3 3.2 2.8 2.1 2.0 1.9 1.9 RT54SX16 3.3 3.3 2.9 2.8 2.5 2.4 2.3 2.3 RT54SX32 3.3 3.3 3.2 3.0 2.6 2.5 2.4 2.2 Ramp Rate Table 3 * Typical Peak Transient Current (mA) 0.33V/s 0.033V/s 6.6V/ms 3.3V/ms 0.66V/ms 0.33V/ms 0.132V/ms 0.033V/ms 54SX08 195 175 82 58 42 42 39 39 54SX16 415 413 274 172 90 70 53 45 54SX16P 369 368 250 145 55 45 30 30 54SX32 732 696 628 469 220 187 169 158 RT54SX16 389 348 276 222 183 159 126 126 RT54SX32 599 600 524 427 285 243 212 167 Ramp Rate Po w e r- U p/ P ow e r - D o w n R e qu i r e m en ts S equen ce The order in which the power supplies are ramped up and ramped down is critical for all 54SX and RT54SX devices. To avoid device damage, VCCR must be greater than or equal to VCCI at all times. This guideline applies to both power-up and power-down sequences. A p-n junction exists between the VCCI and VCCR power supplies in the circuitry of the JTAG TDI and TMS pins. When VCCI is greater than VCCR by 0.6V, the p-n junction will forward bias and cause excessive current flow. This condition cannot be violated for any period of time. The excessive current through the p-n junction may damage the TDI and TMS pin circuitry. Com p let e Po wer -D own On an Actel 54SX16, 54SX16P, RT54SX16, or RT54SX32 FPGA, it is important that VCCA does not rise above 0.0V before initiating the power-up sequence. VCCA must begin its power-up from 0.0V. This power-up consideration does not apply to 54SX08 or 54SX32 devices due to differences in the power-up circuitry. If any amount of residual voltage is present on VCCA before power-up, the sensing circuit may not generate an appropriate reset signal, thus causing the FPGA to power-up in an unknown state. Should the JTAG 2 state machine power-up in an unknown state, all I/Os will most likely be tristated. This does not cause any damage to the device, but the FPGA may not be functional after power-up. For all RT54SX devices, this can be avoided by connecting the TRST pin to ground during power-up. Actel recommends this pin always be connected to ground for normal operation. D ri vi n g I / O s b e f o r e P ow e r -U p As stated earlier, the 54SX and RT54SX I/Os may drive low during power-up. Actel recommends that voltage only be applied to the inputs after the power-up sequence has been completed for all power supplies. Applying a voltage greater than VCCI to the TDI or TMS pins on 54SX and RT54SX devices may cause residual voltage to appear on VCCA. This may cause the symptoms described in the previous section, "Complete Power-Down" for 54SX16, 54SX16P, RT54SX16, and RT54SX32 devices. This is due to a current path that exists from the TDI and TMS pins to VCCA (if it uses the same power supply as VCCI). If VCCA and VCCI are not directly connected together on the board, this Po w e r -U p a n d P o w e r -D o w n B e h a vi o r o f 5 4 S X a n d R T5 4 S X D e v ic e s condition does not occur. External pull-up resistors from these pins to VCCI are acceptable; however, pull-up resistors to VCCR may cause this condition. C on cl u si o n The power-up requirements for 54SX and RT54SX devices are defined in this Application Note. In order to keep the transient current during power-up to a minimum, a VCCA ramp rate of 0.66V/ms or slower is recommended. If all the above power sequencing, prevention of residual VCCA voltage, and I/O driving guidelines are met, the 54SX and RT54SX devices will function correctly after power-up. Specific board level power-up solutions are left to the designer. 3 Actel and the Actel logo are registered trademarks of Actel Corporation. 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