p2125vps20 Renesas Electronics Corporation., p2125vps20 Datasheet - Page 298

• Manufacturer Part Number: p2125vps20
• Description: 16-bit Single-chip Microcomputer H8s Family / H8s/2100 Series
• Manufacturer: Renesas Electronics Corporation.

Section 12 16-Bit Cycle Measurement Timer (TCM)
(5)
Two cycles of the input waveform from a fan form a single measurement cycle in speed
measurement mode. The rotation speed of a fan is given by the following formula.
• When the fan motor generates two pulses per rotation:
(6)
When the TCM is used in speed measurement mode, take the range of error in measurement and
the range of measurable frequencies into account in selecting the source clock to drive counting.
When measuring the pulses of a signal input via the TCMCYI pin, the possible error in the
number counted due to the conditions of measurement is N ± 1 (N: the correct number of input
TCM cycles). Therefore, the error in measurement is given by the following formula.
Table 12.2 lists the ranges of error in measurement when the TCM period is from 60 to 3.75 ms
and the frequency of the system clock φ is from 8 to 20 MHz.
Table 12.2 Range of Error in Measurement
Rev. 1.00 Sep. 21, 2006 Page 260 of 658
REJ09B0310-0100
Input Clock Frequency
of TCMCNT
φ/2
φ/8
φ/16
φ/32
φ/64
φ/128
φ/256
Formula for Calculating Fan Speed from Results in Speed Measurement Mode
Measurement Error and Range in Speed Measurement Mode
Fan rotation speed (RPM) =
Measurement error =
Number Counted by TCMCNT
15000 to 65535
3750 to 65535
1875 to 65535
937±1 to 65535
468±1 to 37500
234±1 to 18750
117±1 to 9375
N ± 1
N
× 100% - 1 =
Number of captures counted by TCMCNT
TCMCNT input clock frequency × 60 × 2
N
1
H'3A98 to H'FFFF
H'EA6 to H'FFFF
H'753 to H'FFFF
H'3A9±1 to H'FFFF
H'1D4±1 to H'927C
H'EA±1 to H'493E
H'75±1 to H'249F
× 100%
Range of Error (%)
0.001 to 0.006
0.001 to 0.002
0.001 to 0.05
0.001 to 0.11
0.002 to 0.21
0.005 to 0.43
0.01 to 0.85