EM78P458AP datasheet(34/76 Pages) EMC

34 / 76 page

EM78P458/459

OTP ROM

3. A/D Sampling Time

The accuracy, linearity, and speed of the successive approximation A/D converter are dependent on

the properties of the ADC and the comparator. The source impedance and the internal sampling

impedance directly affect the time required to charge the sample holding capacitor. The application

program controls the length of the sample time to meet the specified accuracy. Generally speaking,

the program should wait for 1 μs for each K

Ω of the analog source impedance and at least 1 μs for

the low-impedance source. After the analog input channel is selected, this acquisition time must be

done before the conversion can be started.

4. A/D Conversion Time

CKR0 and CKR1 select the conversion time (Tct), in terms of instruction cycles. This allows the MCU

to run at the maximum frequency without sacrificing the accuracy of A/D conversion. For the

EM78P458/459, the conversion time per bit is about 4μs. Table 8 shows the relationship between

Tct and the maximum operating frequencies.

Table 9 Tct vs. the Maximum Operation Frequency

CKR0:CKR1

Operation Mode

Max. operation frequency

Fsco/4

1 MHz

Fsco/16

4 MHz

Fsco/64

16MHz

Internal RC

5. A/D Operation During Sleep Mode

In order to reduce power consumption, the A/D conversion remains operational during sleep mode,

and is obligated to implement the internal RC clock source mode. As the SLEP instruction is executed,

all the operations of the MCU will stop except for the A/D conversion. The RUN bit will be cleared and

the result will be fed to the ADDATA when the conversion is completed. If the ADIE is enabled, the

device will wake up. Otherwise, the A/D conversion will be shut off, no matter what the status of

ADPD bit is.

6. Programming Steps/Considerations

1. Programming steps

Follow these steps to obtain data from the ADC:

(1) Write to the three bits (IMS2:IMS0) on the AD-CMP-CON1 register to define the characteristics

of R6: Digital I/O, analog channels, and voltage reference pin;

(2) Write to the ADCON register to configure AD module:

This specification is subject to change without prior notice.

06.25.2004 (V1.4)

EM78P458AP Datasheet(PDF) 34 Page - ELAN Microelectronics Corp

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