User's Manual
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| Wildcat (BL2000) User's Manual |
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Appendix C. Power Supply
Appendix C describes the power circuitry distributed on the BL2000.
C.1 Power Supplies
Power is supplied to the BL2000 via a mini phone jack located at J7 or through the screw terminal strip, header J2. The BL2000 itself is protected against reverse polarity by a diode at D1 as shown in Figure C-1.
Capacitor C28 provides surge current protection for the voltage regulator, and allows the external power supply to be located some distance away from the BL2000. A switching power regulator is used. The input voltage range is from 9 V to 40 V.
The BL2000 can alternatively be powered by 24 V AC. In this case D1 and C28 act as a half-wave rectifier to produce approximately 40 V DC at the input of the switching regulator, U12. Although a significant drop will be measured at DCIN, the voltage will never drop below +9 V DC. As long as the minimum input level is maintained at the input to the regulator, Vcc will be held at +5 V DC.
Pin 12 on header J9 can be configured to either supply Vcc (0 W surface-mounted resistor installed at R160) or GND (0 W surface-mounted resistor installed at R161). When using pin 12 on header J9 to supply Vcc, take care not to draw more than 25 mA current from this pin, especially if you are using 24 V AC as your +RAW input power supply. The R160 and R161 locations are shown in Figure A-4.
C.1.1 Power for Analog Circuits
Power to the analog circuits is provided by way of a two-stage low-pass filter, which isolates the analog section from digital noise generated by the other components. The analog power voltage +V powers the op-amp for the buffered A/D converter inputs, the A/D converter, and the 4.096 V reference circuit. The two D/A converters can be powered either from the reference, which is the standard, or from +V when ratiometric measurements are desired. The maximum current draw on +V is less than 10 mA.
There are three digital grounds, one on each of the screw-terminal headers associated with the digital functions (J2, J8, and J9). The digital ground and the analog ground share a single split ground plane on the board, with the analog ground connected at a single point to the digital ground by a 0 W resistor (R87). This is done to minimize digital noise in the analog circuits and to eliminate the possibility of ground loops. External connections to analog ground are made on screw-terminal header J4.
C.2 Batteries and External Battery Connections
The SRAM and the real-time clock have battery backup. Power to the SRAM and the real-time clock (VRAM) is provided by two different sources, depending on whether the main part of the BL2000 is powered or not. When the BL2000 is powered normally, and Vcc is within operating limits, the SRAM and the real-time clock are powered from Vcc. If power to the board is lost or falls below 4.63 V, the VRAM power will come from the battery. The reset generator circuit controls the source of power by way of its /RESET output signal.
A replaceable 265 mA·h lithium battery provides power to the real-time clock and SRAM when external power is removed from the circuit board. The drain on the battery is typically less than 10 µA when there is no external power applied, and so the expected in-service life of the battery is
The drain on the battery is typically less than 4 µA when external power is applied, and so the expected BL2000 shelf life is
A long-life 950 mA·h solder-in battery is also provided for in the board layout. Alternatively, an external battery may be connected to the BL2000 via header J12. The existing battery does not have to be removed when an external battery is used.
C.2.1 Replacing the Backup Battery
The battery is user-replaceable, and is fitted in a battery holder. To replace the battery, lift up on the spring clip and slide out the old battery. Use only a Panasonic CR2330 or equivalent replacement battery, and insert it into the battery holder with the + side facing up.
C.2.2 Battery-Backup Circuit
Figure C-2 shows the battery-backup circuit.
The battery-backup circuit serves three purposes:
- It reduces the battery voltage to the SRAM and to the real-time clock, thereby limiting the current consumed by the real-time clock and lengthening the battery life.
- It ensures that current can flow only out of the battery to prevent charging the battery.
- A voltage, VOSC, is supplied to U14, which keeps the 32.768 kHz oscillator working when the voltage begins to drop.
VRAM and Vcc are nearly equal (<100 mV, typically 10 mV) when power is supplied to the BL2000.
C.2.3 Power to VRAM Switch
The VRAM switch, shown in Figure C-3, allows the battery backup to provide power when the external power goes off. The switch provides an isolation between Vcc and the battery when Vcc goes low. This prevents the Vcc line from draining the battery.
Field-effect transistor Q9 is needed to provide a very small voltage drop between Vcc and VRAM (<100 mV, typically 10 mV) so that the board components powered by Vcc will not have a significantly different voltage than VRAM.
When the BL2000 is not in reset, the /RESET line will be high. This turns on Q8, causing its collector to go low. This turns on Q9, allowing VRAM to nearly equal Vcc.
When the BL2000 is in reset, the /RESET line will go low. This turns off Q8 and Q9, providing an isolation between Vcc and VRAM.
C.2.4 Reset Generator
The BL2000 uses a reset generator, U4, to reset the Rabbit 2000 microprocessor when the voltage drops below the voltage necessary for reliable operation. The reset occurs between 4.50 V and 4.75 V, typically 4.63 V. The reset can be initiated either externally or by a watchdog timeout (WDTOUT) on the Rabbit 2000 microprocessor.
C.3 Chip Select Circuit
Figure C-5 shows a schematic of the chip select circuit.
The current drain on the battery in a battery-backed circuit must be kept at a minimum. When the BL2000 is not powered, the battery keeps the SRAM memory contents and the real-time clock (RTC) going. The SRAM has a powerdown mode that greatly reduces power consumption. This powerdown mode is activated by raising the chip select (CS) signal line. Normally the SRAM requires Vcc to operate. However, only 2 V is required for data retention in powerdown mode. Thus, when power is removed from the circuit, the battery voltage needs to be provided to both the SRAM power pin and to the CS signal line. The CS control circuit accomplishes this task for the SRAM's chip select signal line.
In a powered-up condition, the CS control circuit must allow the processor's chip select signal /CS1 to control the SRAM's CS signal /CSRAM. So, with power applied, /CSRAM must be the same signal as /CS1, and with power removed, /CSRAM must be held high (but only needs to be battery voltage high). Q3 and Q4 are MOSFET transistors with complementary polarity. They are both turned on when power is applied to the circuit. They allow the CS signal to pass from the processor to the SRAM so that the processor can periodically access the SRAM. When power is removed from the circuit, the transistors will turn off and isolate /CSRAM from the processor. The isolated /CSRAM line has a 100 kW pullup resistor to VRAM (R101). This pullup resistor keeps /CSRAM at the VRAM voltage level (which under no power condition is the backup battery's regulated voltage at a little more than 2 V).
Transistors Q3 and Q4 are of opposite polarity so that a rail-to-rail voltage can be passed. When the /CS1 voltage is low, Q4 will conduct. When the /CS1 voltage is high, Q3 conducts. It takes time for the transistors to turn on, creating a propagation delay. This propagation delay is typically very small, about 10 ns to 15 ns.
The signal that turns the transistors on is a high on the processor's reset line, /RESET. When the BL2000 is not in reset, the reset line will be high, turning on n-channel Q4 directly and p-channel FET Q3 by way of Q8. When the board is in reset both Q3 and Q4 are off, isolating /CSRAM and allowing it to be pulled to VRAM.
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