| ??? 02/09/05 23:34 Read: times |
#87043 - This heart of mine on fire Responding to: ???'s previous message |
Hi, Eric.
The "sleeping beauty" in your serial ST2221 approach...tell her she's got another admirer! In http://www.toyolite.com.tw/b1.htm we have ST2221A --> Constant Current Output: 5~90mA ST2221C --> Constant Current Output: 5~120mA But in page 1 of both datasheets of ST2221A and ST2221C we have this phrase: Constant Current Output : Current with one resistor for 5mA to 90mA Again in page 5 of both the pdfs we have: Output Current IOUT 90 mA This makes me doubt maybe both of them deliver the same amount of current, even despite what the first link says. Right? Wrong? http://www.allegromicro.com/techpub2/...295031.pdf says: "Multiplexed displays must typically be operated at a current √n times the nominal operating current (where n is the number of digits)". Now in my project, I multiplex 32 columns, so the needed current will be something like 5x[at least]50mA=250mA. High flux LEDs are used:  For each one of the 32x32 LED matrixes I'm gonna use a couple of ST2221Cs as row driver plus 32 PNP xistors driven by a 5x32 decoder made up of two HC154s as column driver. The serial data to the ST2221s on each 32x32 board comes from one of the pins of Pz and Px.y clocks them all. Now considering the need for interdigit blanking I've got two choices: 1) I may use /ENABLE pin of ST2221 to turn off the display or 2) I may use /G pin of HC154s to turn off the display. They both seem to be similar, but I read something from Allegro: This phenomena(ghosting) results from the source driver requiring more time to turn off than the sink driver takes to turn on. Blanking will delay the sink driver turn on and will allow the source driver to turn off completely. This is performed with the OUTPUT ENABLE function. Now, is it correct to conclude that it's better to use /ENABLE of ST2221 instead of /G of HC154? You truely insist on separating VCC and VLED and the GND and V0 justlike what Intel says in "Designing Microcontroller Systems for Electrically Noisy Environments" (AP-125,page 11): Currents in multiplexed LED displays tend to put a lot of noise on the ground and supply lines because of the constant switching and changing involved in the scanning process. The segment driver ground is relatively quiet, since it doesn't conduct the LED currents. The digit driver ground is noisier, and should be provided with a separate path to the PCB ground terminal, even if the PCB ground layout is gridded. The LED feed and return current paths should be laid out on opposite sides of the board like parallel flat conductors. But ST2221 uses only one VDD pin. Do they "internally" separate VCC of the shift register and LED driver inside the pakage or what? You see...I use a ferrite bead plus a handfull of multilayer and tantaluim capacitors to "clean" the power lines to TTL ICs an uC, while LEDs are driven with a rather "raw" VLED. I don't know...maybe I'm just griping instead of finding my "way". Once you talked about rumors going around about 2221C heat dissipation. Also in one of your posts you named ST2221A to be the best choice, Was the emphasis on 2221 or on A? Which one do you suggest 2221A or 2221C? (the Allegro or something one is not likely to be accessible for me, It's a pitty -regardless of the political issues- that I can't even request for a sample from any of Western manufacturers, Iran is not even listed in their country list box! Blast that "Islamic revolution". Hope Candolisa "manages" to realize that the "Ayatollahs" do not necessarily represent us. Sorry for all these questions, I'm just fed up with fooling around the board having soldering iron in one hand and debugging my messaging software with the other while trying to measure voltages with an ohmeter using my bare feet:)  |
