Datasheet

ST72321Rx ST72321ARx ST72321Jx
65/193
ON-CHIP PERIPHERALS (Cont’d)
10.3.3 Register Description
CONTROL / STATUS REGISTER (ARTCSR)
Read/Write
Reset Value: 0000 0000 (00h)
Bit 7 = EXCL
External Clock
This bit is set and cleared by software. It selects the
input clock for the 7-bit prescaler.
0: CPU clock.
1: External clock.
Bit 6:4 = CC[2:0] Counter Clock Control
These bits are set and cleared by software. They
determine the prescaler division ratio from f
INPUT
.
Bit 3 = TCE Timer Counter Enable
This bit is set and cleared by software. It puts the
timer in the lowest power consumption mode.
0: Counter stopped (prescaler and counter frozen).
1: Counter running.
Bit 2 = FCRL
Force Counter Re-Load
This bit is write-only and any attempt to read it will
yield a logical zero. When set, it causes the contents
of ARTARR register to be loaded into the counter,
and the content of the prescaler register to be
cleared in order to initialize the timer before starting
to count.
Bit 1 = OIE
Overflow Interrupt Enable
This bit is set and cleared by software. It allows to
enable/disable the interrupt which is generated
when the OVF bit is set.
0: Overflow Interrupt disable.
1: Overflow Interrupt enable.
Bit 0 = OVF
Overflow Flag
This bit is set by hardware and cleared by software
reading the ARTCSR register. It indicates the tran-
sition of the counter from FFh to the ARTARR val-
ue
.
0: New transition not yet reached
1: Transition reached
COUNTER ACCESS REGISTER (ARTCAR)
Read/Write
Reset Value: 0000 0000 (00h)
Bit 7:0 = CA[7:0] Counter Access Data
These bits can be set and cleared either by hard-
ware or by software. The ARTCAR register is used
to read or write the auto-reload counter “on the fly”
(while it is counting).
AUTO-RELOAD REGISTER (ARTARR)
Read/Write
Reset Value: 0000 0000 (00h)
Bit 7:0 =
AR[7:0]
Counter Auto-Reload Data
These bits are set and cleared by software. They
are used to hold the auto-reload value which is au-
tomatically loaded in the counter when an overflow
occurs. At the same time, the PWM output levels
are changed according to the corresponding OPx
bit in the PWMCR register.
This register has two PWM management func-
tions:
– Adjusting the PWM frequency
– Setting the PWM duty cycle resolution
PWM Frequency vs Resolution:
70
EXCL CC2 CC1 CC0 TCE FCRL OIE OVF
f
COUNTER
With f
INPUT
=8 MHz CC2 CC1 CC0
f
INPUT
f
INPUT
/ 2
f
INPUT
/ 4
f
INPUT
/ 8
f
INPUT
/ 16
f
INPUT
/ 32
f
INPUT
/ 64
f
INPUT
/ 128
8 MHz
4 MHz
2 MHz
1 MHz
500 kHz
250 kHz
125 kHz
62.5 kHz
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
70
CA7CA6CA5CA4CA3CA2CA1CA0
70
AR7AR6AR5AR4AR3AR2AR1AR0
ARTARR
value
Resolution
f
PWM
Min Max
0 8-bit ~0.244 kHz 31.25 kHz
[ 0..127 ] > 7-bit ~0.244 kHz 62.5 kHz
[ 128..191 ] > 6-bit ~0.488 kHz 125 kHz
[ 192..223 ] > 5-bit ~0.977 kHz 250 kHz
[ 224..239 ] > 4-bit ~1.953 kHz 500 kHz