Under steady state (non-transient) conditions, IOL must be externally limited as follows:
Maximum IOL per port pin: 10 mA
Maximum IOL per 8-bit port:
Port 0: 26 mA Ports 1, 2, 3: 15 mA
Maximum total IOL for all output pins: 71 mA
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater
than the listed test conditions.
what means, that you will have to use driver(s) or anythink that can sink (7-segment with common anode) or source (7-segment with common cathode) more current ..
Most likely what is being done is the segments within each 7 Seg display are individually multiplexed. This way only 1 segment is on at any time and the average IOL is only 10 Ma.
They assumed that not all segment would be on at one time (can you imagine situation like 88:88 - of course not), so the combined segments' current could be moretheless within 89C51 max Iol total ..
Also, by using 470Ω curent limiting resistors max current per segment is only ≈5mA ..
But don't expect the segments to shine to brightly ..
I have to disagree with this satements ..
First of all, even if the multiplexing method uses less control lines, but, if you have planty of them available - what's the point???
As far as power consumption is concerned, one has to deliver, on average, certain amount of current to ensure certain level of LED brightness ..
Tell me, what is the difference between, for instance, 5mA DC and 50mA @ 10% PWM?
After all, these 89C51 pins are unable to deliver more than x mA of Iol current ..
Thanks Ianp for replay
but do you think there is no difrence between 5mA and - 50mA for short time with
a verage caculated to be 5mA - at the brightness of leds
Thanks again for all replay
wating for answer
MedTronic
Thanks Ianp for fast replay
you are so helpful , i understand what you mean
another small question
you know the led can't pass more than 25mA,how can i make the led good bright
with multiplexing,how to caculate that
for example if i have 8 leds,i want to scan it what is the curent to thise leds to be
go brightness
thanks again
Thanks Ianp for fast replay
you are so helpful , i understand what you mean
another small question
you know the led can't pass more than 25mA,how can i make the led good bright
with multiplexing,how to caculate that
for example if i have 8 leds,i want to scan it what is the curent to thise leds to be
go brightness
thanks again
You will need data sheets of your LEDs ..
There you will find out what current should be used to obtain a suitable brightness ..
Lets say, you need 10mA per LED ..
As the LEDs are multiplexed you need at least a current of about 80mA / LED @ 12.5% duty cycle (for 8 LEDs), so the average current is: 12.5% *80mA = 10mA ..
Thanks Ianp for replay...but this time you are too late, i wait you too much but
you are so helpful again, i understand what you mean ,cause it what i predect
thanks too much