3850 datasheet(39/101 Pages) RENESAS

39 / 101 page

Rev.3.01

2003.06.20

page 39 of 97

3850 Group (Spec.H/A)

PULSE WIDTH MODULATION (PWM)

The 3850 group (spec. H/A) has a PWM function with an 8-bit

resolution, based on a signal that is the clock input XIN or that

clock input divided by 2.

Data Setting

The PWM output pin also functions as port P44. Set the PWM

period by the PWM prescaler, and set the “H” term of output pulse

by the PWM register.

If the value in the PWM prescaler is n and the value in the PWM

PWM period = 255 ✕ (n+1) / f(XIN)

= 31.875 ✕ (n+1)

µs

(when f(XIN) = 8 MHz,count source selection bit = “0”)

Output pulse “H” term = PWM period ✕ m / 255

= 0.125 ✕ (n+1) ✕ m

µs

(when f(XIN) = 8 MHz,count source selection bit = “0”)

Fig. 35 Timing of PWM period

Fig. 36 Block diagram of PWM function

PWM Operation

When bit 0 (PWM enable bit) of the PWM control register is set to

“1”, operation starts by initializing the PWM output circuit, and

pulses are output starting at an “H”.

If the PWM register or PWM prescaler is updated during PWM

output, the pulses will change in the cycle after the one in which

the change was made.

31.875 ✕ m ✕ (n+1)

255

µs

T = [31.875 ✕ (n+1)]

µs

PWM output

m: Contents of PWM register

n : Contents of PWM prescaler

T : PWM period (when f(XIN) = 8 MHz,count source

selection bit = “0”)

Data bus

Count source

selection bit

“0”

“1”

PWM

prescaler pre-latch

PWM

prescaler latch

PWM

Transfer control circuit

PWM register

1/2

XIN

Port P44 latch

PWM enable bit

Port P44

PWM prescaler

(XCIN at low-speed mode)

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