Intel 8XC196MH, 8XC196MD, 8XC196MC manual Programming the A/D Time Register, Adresult Write

8XC196MC, MD, MH USER’S MANUAL

AD_RESULT (Write)	Address:	1FAAH
	Reset State (MC, MD):	FFC0H
	Reset State (MH):	7FC0H

The high byte of the A/D result (AD_RESULT) register can be written to set the reference voltage for the A/D threshold-detection modes.

15

REFV7	REFV6	REFV5	REFV4
7
—	—	—	—

8

REFV3	REFV2	REFV1	REFV0
			0
—	—	—	—

Bit	Bit	Function
Number	Mnemonic
15:8	REFV7:0	Reference Voltage
		These bits specify the threshold value. This selects a reference voltage
		that is compared with an analog input pin. When the voltage on the
		analog input pin crosses over (detect high) or under (detect low) the
		threshold value, the A/D conversion complete interrupt pending bit is set.
		Use the following formula to determine the value to write to this register
		for a given threshold voltage.
		reference voltage = desired----------------------threshold--------------------------voltage---------------------×-----256--------
		VREF – ANGND
7:0	—	Reserved; for compatibility with future devices, write zeros to these bits.

Figure 12-3. A/D Result (AD_RESULT) Register — Write Format

12.4.3 Programming the A/D Time Register

Two parameters, sample time and conversion time, control the time required for an A/D conver- sion. The sample time is the length of time that the analog input voltage is actually connected to the sample capacitor. If this time is too short, the sample capacitor will not charge completely. If the sample time is too long, the input voltage may change and cause conversion errors. The con- version time is the length of time required to convert the analog input voltage stored on the sample capacitor to a digital value. The conversion time must be long enough for the comparator and cir- cuitry to settle and resolve the voltage. Excessively long conversion times allow the sample ca- pacitor to discharge, degrading accuracy.

The AD_TIME register (Figure 12-4) specifies the A/D sample and conversion times. To avoid erroneous conversion results, use the TSAM and TCONV specifications on the datasheet to determine appropriate values.

12-6