HC05 datasheet(126/232 Pages) MOTOROLA

126 / 232 page

MOTOROLA

8-2

MC68HC05X16

Rev. 1

PULSE LENGTH D/A CONVERTERS

The D/A converter has two data registers associated with it, PLMA and PLMB.

This is a dual 8-bit resolution D/A converter associated with two output pins (PLMA and PLMB).

The outputs are pulse length modulated signals whose duty cycle ratio may be modiﬁed. These

signals can be used directly as PLMs, or the ﬁltered average may be used as general purpose

analog outputs.

The longest repetition period is 4096 times the programmable timer clock period (CPU clock

multiplied by four), and the shortest repetition period is 256 times the programmable timer clock

period (the repetition rate frequencies for a 4 MHz crystal are 122 Hz and 1953 Hz respectively).

Registers PLMA ($0A) and PLMB ($0B) are associated with the pulse length values of the two

counters. A value of $00 loaded into these registers results in a continuously low output on the

corresponding D/A output pin. A value of $80 results in a 50% duty cycle output, and so on, to the

maximum value $FF corresponding to an output which is at ‘1’ for 255/256 of the cycle. When the

MCU makes a write to register PLMA or PLMB the new value will only be picked up by the D/A

converters at the end of a complete cycle of conversion. This results in a monotonic change of the

DC component at the output without overshoots or vicious starts (a vicious start is an output which

gives totally erroneous PLM during the period immediately following an update of the PLM D/A

registers). This feature is achieved by double buffering of the PLM D/A registers. Examples of

PWM output waveforms are shown in Figure 8-2.

Address

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

State

on reset

Pulse length modulation A (PLMA)

$000A

0000 0000

Pulse length modulation B (PLMB)

$000B

0000 0000

Figure 8-2 PLM output waveform examples

256 T

255 T

128 T

$80

$FF

T = 4 CPU clocks in fast mode and 64 CPU clocks in slow mode

128 T

$00

$01

255 T

124

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