Datasheet

V850ES/JG3 CHAPTER 10 WATCH TIMER FUNCTIONS
R01UH0015EJ0300 Rev.3.00 Page 391 of 870
Sep 30, 2010
10.4 Operation
10.4.1 Operation as watch timer
The watch timer generates an interrupt request signal (INTWT) at fixed time intervals. The watch timer operates using
time intervals of 0.25 or 0.5 seconds with the subclock (32.768 kHz) or main clock.
The count operation starts when the WTM.WTM1 and WTM.WTM0 bits are set to 11. When the WTM0 bit is cleared to
0, the 11-bit prescaler and 5-bit counter are cleared and the count operation stops.
The time of the watch timer can be adjusted by clearing the WTM1 bit to 0 and then the 5-bit counter when operating at
the same time as the interval timer. At this time, an error of up to 15.6 ms may occur for the watch timer, but the interval
timer is not affected.
If the main clock is used as the count clock of the watch timer, set the count clock using the PRSM0.BGCS01 and
BGCS00 bits, the 8-bit comparison value using the PRSCM0 register, and the count clock frequency (f
BRG) of the watch
timer to 32.768 kHz.
When the PRSM0.BGCE0 bit is set (1), f
BRG is supplied to the watch timer.
f
BRG can be calculated by the following expression.
f
BRG = fX/(2
m+1
× N)
To set f
BRG to 32.768 kHz, perform the following calculation and set the BGCS01 and BGCS00 bits and the PRSCM0
register.
<1> Set N = f
X/65,536. Set m = 0.
<2> When the value resulting from rounding up the first decimal place of N is even, set N before the roundup as N/2
and m as m + 1.
<3> Repeat <2> until N is odd or m = 3.
<4> Set the value resulting from rounding up the first decimal place of N to the PRSCM0 register and m to the BGCS01
and BGCS00 bits.
Example: When f
X = 4.00 MHz
<1> N = 4,000,000/65,536 = 61.03…, m = 0
<2>, <3> Because N (round up the first decimal place) is odd, N = 61, m = 0.
<4> Set value of PRSCM0 register: 3DH (61), set value of BGCS01 and BGCS00 bits: 00
At this time, the actual f
BRG frequency is as follows.
fBRG = fX/(2
m+1
× N) = 4,000,000/(2 × 61)
= 32.787 kHz
Remark m: Division value (set value of BGCS01 and BGCS00 bits) = 0 to 3
N: Set value of PRSCM0 register = 1 to 256
However, N = 256 only when PRSCM0 register is set to 00H.
f
X: Main clock oscillation frequency