Philips P89LPC901, P89LPC903, P89LPC902 user manual Comparator Configuration, CMPn

Philips Semiconductors	User’s Manual - Preliminary -
ANALOG COMPARATORS	P89LPC901/902/903
10. ANALOG COMPARATORS

One analog comparator is provided on the P89LPC901 and two analog comparators are provided on both the P89LPC902 and P89LPC903 . Comparator operation is such that the output is a logical one when the positive input is greater than the negative input (selectable from a pin or an internal reference voltage). Otherwise the output is a zero. The output may be read in a register. On the P89LPC902 the output may also be routed to a pin. The comparator(s) may be configured to cause an interrupt when the output value changes.

The connections to the comparator(s) are shown in Figure 10-2 - Figure 10-4. The comparator functions to VDD = 2.4V.

When the comparator is first enabled, the comparator output and interrupt flag are not guaranteed to be stable for 10 microseconds. The comparator interrupt should not be enabled during that time, and the comparator interrupt flag must be cleared before the interrupt is enabled in order to prevent an immediate interrupt service.

Comparator Configuration

The comparator(s) have a control register(s), CMPn, and is shown in Figure 10-1. The possible configurations for the comparator are shown in Figure 10-5.

CMPn
Address: ACh		7		6	5	4	3	2	1	0
Not bit addressable			-	-	CEn	-	CNn	OEn	COn	CMFn
Reset Source(s): Any reset
Reset Value: xx000000B
BIT	SYMBOL	FUNCTION
CMP.7, 6	-	Reserved for future use.
CMP.5	CEn	Comparator enable. When set, the comparator function is enabled. Comparator output is
		stable 10 microseconds after CEn is set.
CMP.4	-	Reserved for future use.
CMP.3	CNn	Comparator negative input select. When 0, the comparator reference pin CMPREF is
		selected as the negative comparator input. When 1, the internal comparator reference,
		Vref, is selected as the negative comparator input.
CMP.2	OEn	Output enable. When 1, the comparator output is connected to the CMPn pin if the
		comparator is enabled (CEn = 1). This output is asynchronous to the CPU clock.
CMP.1	COn	Comparator output, synchronized to the CPU clock to allow reading by software.
CMP.0	CMFn	Comparator interrupt flag. This bit is set by hardware whenever the comparator output
		COn changes state. This bit will cause a hardware interrupt if enabled. Cleared by
		software.

Figure 10-1: Comparator Control Registers (CMP1 and CMP2)

2003 Dec 8 75