AS3687 datasheet(29/55 Pages) AMSCO

29 / 55 page

AS3687/87XM

Datasheet

www.austriamicrosystems.com/AS3687

1v3-4

28 - 54

Figure

20 –LED Pattern timing

pattern_slow pattern_delay2 pattern_delay[1..0]

bit duration [ms]

delay [s]

pattern duration [s]

delay between patterns

pattern_color=0 pattern_color=1

between

patterns

(total cycle time:

pattern + delay)

250

800

250

800

250

800

250

800

250

800

250

800

250

800

7.3.4 PWM Generator

The PWM generator can be used for any current sink (CURR1, CURR2, CURR3x, CURR6). The setting applies

for all current sinks, which are controlled by the pwm generator (e.g. CURR1 is pwm controlled if curr1_mode =

10). The pwm modulated signal can switch on/off the current sinks and therefore depending on its duty cycle

change the brightness of an attached LED.

7.3.4.1 Internal PWM Generator

The internal PWM generator uses the 2MHz internal clock as input frequency and its dimming range is 6 bits

digital (2MHz / 2^6 = 31.3kHz pwm frequency) and 2 bits analog. Depending on the actual code in the register

‘pwm_code’ the following algorithm is used:

If pwm_code bit 7 = 1

Then the upper 6 bits (Bits 7:2) of pwm_code are used for the 6 bits PWM generation, which controls the

selected currents sinks directly

If pwm_code bit 7 =0 and bit 6 = 1

Then bits 6:1 of pwm_code are used for the 6 bits PWM generation. This signal controls the selected current

sinks, but the analog current of these sinks is divided by 2

If pwm_code bit 7 and bit 6 = 0

Then bits 5:0 of pwm_code are used for the 6 bits PWM generation. This signal controls the selected current

sinks, but the analog current of these sinks is divided by 4

Figure

21 – PWM Control

pwm_code

7 6 5 4 3 2 1 0

6 bit PWM

to current sink(s)

6 bit PWM

to current sink(s) but analog

currents are divided by 2

6 bit PWM

to current sink(s) but analog

currents are divided by 4

Automatic Up/Down Dimming

If the register pwm_dim_mode is set to 01 (up dimming) or 10 (down dimming) the value within the register

pwm_code is increased (up dimming) or decreased (down dimming) every time and amount (either 1/4

th or 1/8th)

defined by the register pwm_dim_speed. The maximum value of 255 (completely on) and the minimum value of

0 (off) is never exceeded. It is used to smoothly and automatically dim the brightness of the LEDs connceted to

any of the current sinks. The PWM code is readable all the time (Also during up and down dimming)

The waveform for up dimming looks as follows (cycles omitted for simplicity):

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