ZBasic System Library
240
ZBasic Microcontrollers
Type
Subroutine
Invocation
PutDAC(pin, dacValue, dacAccumulator)
PutDAC(pin, dacValue, dacAccumulator, cycles)
Parameter
Method
Type
Description
pin
ByVal
Byte
The output pin.
dacValue
ByVal
numeric
The desired output value. See discussion below.
dacAccumulator
ByRef
Byte
A value used in the DAC process. See the
discussion below.
cycles
ByVal
Byte
The number of PWM cycles to perform.
Discussion
This routine creates a digital approximation of an analog signal on the specified pin using a pseudo-PWM
technique. When called, the specified pin is made an output, a pulse train is generated having an
average value equal to the dacValue parameter and then, after a fixed number of iterations, the pin is
placed in the high impedance input state. If the output is filtered with a low pass filter, the voltage will,
immediately after the process is completed, be at a level between zero and the processor voltage (usually
+5 volts). However, the voltage will begin to decay at a rate dependent on the load presented to the filter.
The voltage can be refreshed from time to time by calling PutDAC() again.
The dacValue parameter may be specified by a Single value or an integral value. If a Single value is
supplied, it should be in the range 0.0 to 1.0 corresponding to the output range of 0 to the processor
voltage (usually +5 volts). If an integral value is supplied, it should be in the range of 0 to 255
corresponding to the same output voltage range as above.
The dacAccumulator parameter is required to ensure continuity between successive calls to
PutDAC(). The value of the parameter after the call should not be modified and the same parameter
should be supplied on the next call. The initial value of the parameter is of no consequence. If your
application uses PutDAC() to create an analog voltage on more than one pin at a time, a separate
accumulator value must be used for each one.
If the cycles parameter is not specified, a single PWM cycle is performed. Each cycle will generate a
burst of pulses for about 3000 CPU cycles (200µS at 14.7MHz) during which time interrupts will be
disabled. At the end of each cycle, the pin is put in high impedance mode and interrupts are re-enabled.
The process is then repeated if the cycle count is greater than one. A cycle count of zero causes no
cycles to be performed.
The selection of components for the required filter depends on several factors. A larger capacitor will
allow the voltage to hold longer but also takes longer to bring up to the proper voltage. As a rule of
thumb, the product of the resistance (in ohms) and the capacitance (in farads) should be on the order of
the number of cycles times one-fourth of the cycle time specified above. For example, with a 100O
resistor and a 1µF capacitor, the cycle count should probably be 2 in order to bring the capacitor up to the
desired voltage level.
For best results, you should probably follow the filter with a high impedance buffer such as a unity gain op
amp circuit, an example of which is shown below. The op amp chosen is not particularly critical, nearly
any will do the job.
For ZBasic devices based on the ATxmega, a hardware DAC is available. In most applications requiring
a DAC, using the hardware DAC will produce much better results.
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