Motorola MCF5281, MCF5282 user manual Interrupt Operation, Error Resulting from Leakage

Models: MCF5282 MCF5281

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Interrupts

27.9.7.2Error Resulting from Leakage

A series resistor limits the current to a signal; therefore, input leakage acting through a large source impedance can degrade A/D accuracy. The maximum input leakage current is specified in MCF5282 Electrical Specifications. Input leakage is greater at higher operating temperatures. In the temperature range from 125°C to 50°C, the leakage current is halved for every 8°C to 12°C reduction in temperature.

Assuming VRH–VRL= 5.12 V, 1 count (with 10-bit resolution) corresponds to 5 mV of input voltage. A typical input leakage of 200 nA acting through 10 kΩ of external series resistance results in an error of 0.4 count (2.0 mV). If the source impedance is 100 kΩ and a typical leakage of 100 nA is present, an error of 2 counts (10 mV) is introduced.

In addition to internal junction leakage, external leakage (for example, if external clamping diodes are used) and charge sharing effects with internal capacitors also contribute to the total leakage current. Table 27-25illustrates the effect of different levels of total leakage on accuracy for different values of source impedance. The error is listed in terms of 10-bit counts.

CAUTION

Leakage below 200 nA is obtainable only within a limited temperature range.

Table 27-25. Error Resulting from Input Leakage (IOff)

Source Impedance

 

Leakage Value (10-Bit Conversions)

 

 

 

 

 

 

 

100 nA

 

200 nA

500 nA

 

1000 nA

 

 

 

 

 

 

 

 

 

 

1 kΩ

 

0.1 counts

 

0.2 counts

 

 

 

 

 

 

 

10 kΩ

0.2 counts

 

0.4 counts

1 counts

 

2 counts

 

 

 

 

 

 

 

100 kΩ

2 counts

 

4 count

10 counts

 

20 counts

 

 

 

 

 

 

 

27.10 Interrupts

The four interrupt lines are outputs of the module and have no priority or arbitration within the module.

27.10.1 Interrupt Operation

QADC inputs can be monitored by polling or by using interrupts. When interrupts are not needed, the completion flag and the pause flag for each queue can be monitored in the status register (QASR0). In other words, flag bits can be polled to determine when new results are available.

Table 27-26shows the status flag and interrupt enable bits which correspond to queue 1 and queue 2 activity.

MOTOROLA

Chapter 27. Queued Analog-to-Digital Converter (QADC)

27-75

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Motorola MCF5281, MCF5282 Interrupt Operation, Error Resulting from Leakage, Error Resulting from Input Leakage IOff