Hi,

the connection between caffeine and the frequency limit of flicker
detection of the retina fascinates me (I believe it and I think I had
experienced it). Can you give me some reference?

Thanks in advance,
Imre


On Thu, 25 Jun 1998, James Cameron wrote:

> G'day,
>
> Amateur wants to learn.
>
> My MOSFETs are getting hot and I'm wondering why.  Is my PWM frequency
> too high?  Am I simply pushing them too hard?
>
> I'm driving a pair of 21W 12V (1.75A) lamps using a MTP3055E MOSFET each
> from port A bits zero and one using a pulse width modulation (PWM)
> routine.  At an ambient temperature of 120C they reach 44.40C, with no
> heatsink or fan.
>
> Circuit is port to gate, source to ground, drain to lamp, then lamp to
> the 13.6V 2A unregulated supply.  I have a GIF of the schematic I can
> mail to anyone who asks.  It's a project for Technical Aid to the
> Disabled ... a light box communication device for very young children.
>
> The supply voltage stabilises under load to be about 14.4V, and a DVM
> across the lamp shows about 10.5V at my chosen duty cycle of 200/256, or
> 71%.
>
> The PWM routine "light" is called as frequently as possible; usually
> with only two instruction cycles between each call.  The PWM is simple
> and is derived from the Scott Edwards source book; repetitive addition
> in an accumulator and taking the carry output as the PWM signal.
>
> The code follows.  I calculate 15 cycles at 4MHz between each setting of
> the output bits.  With a 71% bit pattern of 011101111011101111 that's
> roughly 67 cycles between pulses, so a frequency of 15kHz.  I hope.
>
> a_left  equ     0x12            ; duty cycle accumulators
> a_right equ     0x13
>
> d_left  equ     0x14            ; duty cycle settings (set to 200)
> d_right equ     0x15
>
> port    equ     PORTA
>
> b_left  equ     0x00            ; gate of mosfet for left lamp
> b_right equ     0x01            ; gate of mosfet for right lamp
>
> [...]
>
> light                           ; update lights using duty cycles
>         movf    d_left,w
>         addwf   a_left,f
>         btfsc   STATUS,C
>         goto    light_1
>         bcf     port,b_left
>         goto    light_2
> light_1
>         bsf     port,b_left
> light_2
>         movf    d_right,w
>         addwf   a_right,f
>         btfsc   STATUS,C
>         goto    light_3
>         bcf     port,b_right
>         goto    light_4
> light_3
>         bsf     port,b_right
> light_4
>         return
>
> [...]
>
> --
> James Cameron                              (james.cameron@digital.com)
> Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800
>
>