I see that you already have a couple of solid repliesarader wrote: I'm kicking around the idea of creating a 64x64 led matrix using smaller 8x8 led matrices. My plan is to use shift registers and darlington arrays in order to control the matrix.
I tend to agree, but don't forget that SPI can be treated as a chain of SRs. I do think that you should try to limit the length of the chain. Your idea of a backpack controlling each 8x8 display has been implemented successfully by others and has some advantages. In particular, modularity. If your final intention is a 64x64 display, there may be other configurations. For example, one might multiplex 8 rows of 64 bits at a time, thus reducing the duty cycle. The downside is the need for more SRs. Since you are a software person, you might prefer to reduce the H/W complexity in favor of additional S/W.Philba wrote:I still think a long chain of SRs will actually be your best approach but it's your project. Do read the other threads, there's lots of good info there.
arader wrote:I'm going to have to give it some more thought (obviously) so for now I'm planning on just creating 1 backpack for one of the 8x8 matrices. We'll see how it goes from there.
one quick question however: with SPI, is it possible / allowed to enable the slave select line on more than one slave? I'm just thinking of ways to implement a sort of "broadcast" to a bunch of slaves.
I'll be using the module found here: http://www.futurlec.com/LED/LEDMS88R.shtml - The forward current for each LED is 25ma.bigglez wrote:please post the
part number of the LEDs that you plan to use.
Andybigglez wrote:What is your first name?
Are you suggesting these replace the 74HC595 SRs? it appears the TPIC6C595s only go up to 250ma, but if I couple my 74HC595 with the ULN2803A transistor array I'd have up to 500ma. At 8 leds per row * 25ma each I'd be at 200ma, which is pretty close to 250 but a ways away from 500. The only thing I'm worried about is the column's SR. The datasheet (http://focus.ti.com/lit/ds/symlink/sn74hc595.pdf) has the following:Philba wrote:the power SRs - TPIC6C595 are a good choice.
Input clamp current, IIK (VI < 0 or VI > VCC) (see Note 1) . . . . . . . . . . ±20 mA
Output clamp current, IOK (VO < 0 or VO > VCC) (see Note 1) . . . . . . . . . . ±20 mA
Continuous output current, IO (VO = 0 to VCC) . . . . . . . . . . . . . . . . . ±35 mA
Continuous current through VCC or GND . . . . . . . . . . . . . . . . . . . . . ±70 mA
I'm definitely trying to get the calculations right the first time, but I should remind you it's 64 LEDs/row total, not 512. I'll be hoping for an 8x8 matrix of 8x8 matricies (for 64x64 total leds).Philba wrote:Also, you need to do current and power calculations for your drivers. at 512 LEDs/row, you will probably need a fair amount of current drive. Don't forget the 1/8 duty cycle in your calcs.
I take it this really means shouldn't have more than a 1/32 duty cycle, right?gmarsh wrote:If you want a decently bright display, don't do more than 32 rows.