Circuit provides simpler power-supply-sequence testing
New circuit improves on an earlier power-sequence-testing idea.
Dan Karmann, DLK Engineering, Thornton, CO; Edited by Martin Rowe and Fran Granville -- EDN, December 3, 2009
A previous Design Idea (Reference 1)describes a three-IC control circuit for testing the power sequencing of an SOC (system on chip). Although that circuit lets you control the power-on sequence of two linear power supplies, it uses one eight-pin IC, two 14-pin ICs, several discrete components, and a DPDT (double-pole/double-throw) switch for the control. Replacing most of those components with an inexpensive, eight-pin microcontroller simplifies power-supply-sequencing control because the approach requires less wiring for component interconnections. The approach is also more versatile because it involves only simple changes to the controlling firmware. The simplicity and versatility come at approximately the same component cost.
A disadvantage of this circuit compared with the original is that it requires the appropriate firmware for the microcontroller, an Atmel ATtiny13. However, free tools are available that let you develop and program the microcontroller. This Design Idea includes the source code for the operation of the sequencer in both Basic and C. You can download the code from Listing 1.
The demo version of the Bascom-AVR Basic compiler is fully functional and code-limited to 4 kbytes—four times the code space in the ATtiny13—and is freely downloadable for noncommercial development from MCS Electronics. The WinAVR tools used in this Design Idea use the GNU GCC C/C++, a fully functional, free open-source GNU GCC compiler. You can easily integrate the WinAVR compiler into the free IDE (integrated development) AVR Studio from Atmel. To program the Atmel microcontrollers, you can use free software tools through the microcontroller’s six-pin SPI (serial-programming interface). You can download the easy-to-use, free PonyProg software from Lancos and also obtain the schematics for the programming circuits.
The circuit in Figure 1, like the circuit in Reference 1, includes two TPS75501 regulators, IC2 and IC3. This new circuit needs only IC1, an eight-pin microcontroller; S1, an SPST (single-pole/single throw) pushbutton switch to start the sequence; S2, an SPST toggle switch, or a two-pin header with a jumper, to control the sequence order; and potentiometer R3 to control the sequence delay. According to the firmware in Listing 1, pressing S1 when S2 is open causes the microcontroller to first turn on the 1.5V power supply and then turn on the 3.3V power supply following a delay that potentiometer R3 controls. Pressing S1 when switch S2 is closed causes the microcontroller to first turn on the 3.3V power supply and then turn on the 1.5V power supply following a delay that potentiometer R3 controls. As with the original Design Idea, a second press of S1 causes the power supplies’ turn-off to take place in the same sequence and with the same delay as their turn-on. This scenario provides an opportunity for an easy enhancement or change in operation.
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The voltage level on Pin 7 of IC1 determines the delay, under firmware control, between turning on or off the first and the second power supply. The microcontroller reads this delay voltage with its 10-bit ADC and uses the value to determine the delay according to the following equation: Delay=(VDELAY/VCC)×1024×1 msec, where VDELAY is the delay voltage. This equation yields a delay range from a few microseconds to a bit more than 1 second. As an example, if the delay-voltage value from R3 is the midwiper value of 2.5V, the sequencing delay is approximately 512 msec: (2.5/5V)×1024×1 msec. The delay value is approximate because the microcontroller uses its internal 9.6-MHz RC oscillator to generate the timing with a simple firmware delay loop.
The code in Listing 1 follows the original Design Idea in that a second press of trigger switch S1 causes the power supplies to turn off in the same sequence and with the same delay with which they turn on. The listing includes a constant off_sequence that you can change to change the turn-off sequence with the second press of S1 (Figure 2). This constant OFF_SEQUENCE is currently SEQUENCE_same to operate as the original Design Idea did, but if you set the OFF_SEQUENCE to SEQUENCE_REVERSE, the turn-off sequence will be in the opposite order of the turn-on sequence. Alternatively, if you set the constant off_SEQUENCE to SEQUENCE_none, both power supplies will turn off at once. This feature exemplifies the versatility of this follow-on Design Idea with a simple firmware change. Because the circuit uses only about half the code space in the ATtiny13, you could easily add other desired changes. Although this circuit uses an Atmel microcontroller, you can use almost any low-pin-count microcontroller with a built-in ADC. However, other brands may not have the readily available free development tools that exist for Atmel devices.
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