Freescale Semiconductor DRM079 manual #63, Mtimmod, ACMPSCACF, Acmpsc

Temperature Sensor Measurement

Table 3-2. RC Charging Profile Against Timer Count

Time (∝s)	Voltage across the	ADC Readout
	Temperature Sensor (V)	(Timer Count)
0	0	0
2	0.10	1
4	0.19	2
6	0.28	3
	and so on...
86	2.82	43
88	2.87	44
90	2.91	45
	and so on...
126	3.52	63

Table 3-2shows the entire dynamic range of the temperature sensor voltage can be covered by about 44 timer counts. For convenience, the timer overflow period is set to 63, which is identical to the size of the paging window ($00C0 to $00FF) in the MC9RS08KA2. The timer value captured can be used directly as an index to the paging window for the target PWM period value lookup.

The code below shows how the timer value is captured using RS08 instructions.

ReadSensor:	#(MTIM_BUS_CLKMTIM_DIV_8), MTIMCLK; Change Timer resolution
mov
mov	#63, MTIMMOD	; OF period
mov	#(mMTIMSC_TRSTmMTIMSC_TOIE), MTIMSC; Reset and Start Timer
mov	#(mACMPSC_ACMEmACMPSC_ACIEACMP_OUTPUT_RAISING), ACMPSC
bset	ACMPSC_ACF, ACMPSC	; Enable ACMP, start RC rise
		; Clear ACMP Flag
wait	ACMPSC_ACF, ACMPSC, NoReading
brclr		; Capture timer count
mov	MTIMCNT, SensorReading
bset	ACMPSC_ACF, ACMPSC	; Clear ACMP Flag
clr	ACMPSC	; disable ACMP
wait		; delay to OF and make the
mov		; read process deterministic
	#(mMTIMSC_TSTPmMTIMSC_TRST), MTIMSC; mask interrupt and clear
mov		; flag
	#(MTIM_BUS_CLKMTIM_DIV_256), MTIMCLK; Reset Timer resolution
rts
NoReading:	#$00, SensorReading	; Smallest Number
mov
clr	ACMPSC	; disable ACMP
mov	#(mMTIMSC_TSTPmMTIMSC_TRST), MTIMSC ; mask interrupt and clear
mov		; flag
	#(MTIM_BUS_CLKMTIM_DIV_256), MTIMCLK; Reset Timer resolution
rts
	Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0
Freescale Semiconductor		19