Digi BL1800 user manual 2 DA0, Typical DA0 Voltages for Various Duty Cycles

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3.4.2 DA0

The op amp supporting DA0 translates a 12%–88% duty cycle to an analog voltage range of 0 V to 3 V. The software operates only within this duty cycle; a duty cycle less than 12% is rounded down to 0%, and any duty cycle above 88% is rounded up to 100%.

DA0 uses a voltage divider that consists of R21 and R27 and a gain-offset circuit that con- sists of R26 and R29 to achieve the output range of 0 V to 3 V within the software duty cycle.

The DA0 output can be less than 100 mV for a 0% duty cycle and above 3.0 V for a 100% duty cycle. The duty cycle is programmed as the high-time count of 1024 total counts of the Rabbit 2000’s timer B. Thus, 256 counts would be 25% of 1024 counts, and corre- sponds to a 25% duty cycle.

Table 3 lists typical DA0 voltages measured for various duty cycle values with a load larger than 1 MΩ.

Table 3. Typical DA0 Voltages for Various Duty Cycles

	Duty Cycle	Voltage	Programmed Count
	(%)	(V)	Programmed Count
	(%)	(V)

	0	0.074	0–122

	12	0.076	123

	25	0.530	256

	50	1.467	512

	75	2.406	768

	88	2.875	901

	100	3.345	902–1024

It is important to remember that the DA0 output voltage will not be realized instantaneously after programming in a value. There is a settling time because of the RC time constant (R21 R27 × C20), which is 7.68 ms. For example, the voltage at any given time is

V = VP – (VP – VDA0)e(-t/RC)

(EQ 2)

where V is the voltage at time t, VP is the programmed voltage, VDA0 is the last DA0 out- put voltage from the D/A converter, and RC is the time constant (7.68 ms). The settling will be within 99.326% (or within about 21 mV for a 3 V change in voltage) after five time constants, or 38 ms. Six time constants, 46 ms, will allow settling to within 99.75% (or to within about 8 mV for a 3 V change in voltage). Seven time constants, 54 ms, will allow settling to within 99.91% (or to within about 3 mV for a 3 V change in voltage).

The settling time is reduced somewhat by precharging capacitor C20 with pulse-width modulation from PD2.

26	Jackrabbit (BL1800)

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Contents Jackrabbit BL1800 Programmable Single-Board ComputerTrademarks Digi International IncTable of Contents Index Schematics Features IntroductionJackrabbit Features Development and Evaluation ToolsHow to Use This Manual Additional Product InformationOnline Documentation CE Compliance ImmunityEmissions Safety Design Guidelines GeneralInterfacing the Jackrabbit to Other Devices Jackrabbit BL1800 Development Kit Contents Getting StartedDevelopment Hardware Connections Board Attach Jackrabbit to Prototyping BoardConnect Programming Cable Assemble AC AdapterConnect Power Installing Dynamic C Troubleshooting Run a Sample ProgramWhere Do I Go From Here? Real-Time ClockTechnical Support Subsystems BL1800Jackrabbit Pinouts HeadersDigital Inputs/Outputs Digital InputsDigital Outputs HV3 Sourcing OutputConfigurable High-Current Output Bidirectional I/O Changing HV3 to a Sinking OutputSchematic Diagram of A/D Converter A/D ConverterJackrabbit BL1800 Schematic Diagram of D/A Converters D/A ConvertersV ⋅ 1 e RC 1 DA1Typical DA1 Voltages for Various Duty Cycles 2 DA0 Typical DA0 Voltages for Various Duty CyclesUser’s Manual Serial Communication 1 RS-2322 RS-485 Ground recommended Programming Port 485 R16User’s Manual Changing Between Program Mode and Run Mode Programming CableMemory SramFlash Eprom Other Hardware External InterruptsClock Doubler Spectrum Spreader Jackrabbit BL1800 Software Reference An Overview of Dynamic CJackrabbit BL1800 Sample Programs Jackrabbit Sample ProgramsDEMOJR1.C Sample Program DEMOJR1.CSingle-Stepping Watch Expression Break PointSummary of Features Editing the ProgramWatching Variables Dynamically User’s Manual Other Sample Programs Illustrating Digital I/O R/W pin and DB0-DB3 on 3 RS-232 Serial Communication Sample Programs 4 RS-485 Serial Communication Sample Program Cooperative Multitasking Int vswitch Advantages of Cooperative Multitasking Jackrabbit Function Calls 1 I/O DriversVoid digOutint channel, int value Void anaOutint channel, int value Void anaInint channel, int *value Serial Communication Drivers Upgrading Dynamic C Patches and Bug FixesAdd-On Modules Appendix A. Specifications Electrical and Mechanical Specifications Figure A-1shows the mechanical dimensions for the JackrabbitTable A-1. Jackrabbit Board Specifications Exclusion Zone Exclusion ZonesFigure A-3. User Board Footprint for Jackrabbit Jumper Configurations Figure A-4. Location of Jackrabbit Configurable PositionsTable A-2. Jackrabbit Jumper Configurations Conformal Coating Conformally coated areaUse of Rabbit 2000 Parallel Ports JackrabbitTable A-3. Jackrabbit Pinout Configurations PD0 PD1 Jackrabbit BL1800 Appendix B. Prototyping Board Jackrabbit Connectors User LEDs Buzzer Prototyping Board OverviewPrototyping Board Features Mechanical Dimensions and Layout Top SideUsing the Prototyping Board Top SideDemonstration Board RelayExisting Prototyping Board Top SidePE0 VCC HV0 SM1 SM0 HV2 Stat Jackrabbit BL1800 Appendix C. Power Management Power SuppliesDcin Current mA Batteries and External Battery Connections 950 mA·h = 5.4 years 20 µABattery Backup Circuit Figure C-5shows the Jackrabbit battery backup circuitryPower to Vram Switch Reset GeneratorFigure C-7shows a schematic of the chip select circuit Chip Select CircuitJackrabbit BL1800 Index SMODE0 SMODE1RABDB01.C RABDB02.C Schematics