MSP430 datasheet(27/88 Pages) TI

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7.3.1.5 Programmable Slew Rate

The slew rate feature controls the rate at which the output voltage or current changes. This feature is disabled

by default and is enabled by writing a logic 1 to the DAC_CFG.SR_EN bit. With the slew rate control feature

disabled, the output changes smoothly at a rate limited by the output drive circuitry and the attached load.

With this feature enabled, the output does not slew directly between the two values. Instead, the output steps

digitally at a rate defined by DAC_CFG.SR_STEP[2:0] and DAC_CFG.SR_CLK[2:0]. SR_CLK defines the rate

at which the digital slew updates. SR_STEP defines the amount by which the output value changes at each

update. Section 7.6.1 shows different settings for SR_STEP and SR_CLK.

The time required for the output to slew is expressed as Equation 6:

SlewTime= DeltaCodeChange

SlewStep×SlewClockRate

(6)

where

• Slew Time is expressed in seconds

• Slew Step is controlled by DAC_CFG.SR_STEP

• Slew Clock Rate is controlled by DAC_CFG.SR_CLK

When the slew-rate control feature is enabled, the output changes at the programmed slew rate. This

configuration results in a staircase formation at the output. If the clear code is asserted (see Section 7.3.1.6), the

output slews to the DAC_CLR_CODE value at the programmed slew rate. When new DAC data are written, the

output starts slewing to the new value at the slew rate determined by the current DAC code and the new DAC

data. The update clock frequency for any given value is the same for all output ranges. The step size, however,

varies across output ranges for a given value of step size because the LSB size is different for each output

range.

Two slew-rate control modes are available: linear (default) and sinusoidal. Figure 7-3 and Figure 7-4 show the

typical rising and falling DAC output waveforms, respectively.

4 mA (0x0BA3) to 24 mA (0xF45D) measured on a 40-Ω shunt

Figure 7-3. Linear Slew Rate: Rising

24 mA (0xF45D) to 4 mA (0x0BA3) measured on 40-Ω shunt

Figure 7-4. Linear Slew Rate: Falling

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SLASF21 – DECEMBER 2022

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