0
\$\begingroup\$

Morning All, I've kind of designed myself into a corner using 5 74HC461 into a DMA transfer function. I used these chips initially as they offloaded alot of the mundane address HSB-LSB latching overhead from the 87c52 only to find the HC/HCT version of this chip was never released. The PCB art is done ready for making, software is done, yada yada so I'm trying not to have a drastic redesign slap in the face. What surprises me is there are no CMOS 8 BIT UP COUNTERS WITH PRELOAD AND TRISTATE OUTPUTS's. So I'm looking at a 24pin skinny PIC doing the job. Or its daughter board time. The PCB doesn't have room for 74c161s so a redesign of the board requires use of the existing real estate. Now,can the ports of the pic be put into a high impedance mode like on the 8051's? A(sudo tristate)

\$\endgroup\$
1
  • 1
    \$\begingroup\$ It is very unclear what you're asking here. If you just want to know if PICs support high-impedance outputs, yes, they do; I believe they call it 'tristating' the output. \$\endgroup\$ – Hearth May 2 '17 at 18:48
1
\$\begingroup\$

You should be able to do this with a GAL22V10 - or, at least, the Atmel ATF22V10 as Lattice no longer make their version. Here's the pinout of the 74LS461...

enter image description here

...and here's the ATF22V10:

enter image description here

The macrocell for the 22V10 looks like this...

enter image description here

...so you can use pin 13 as an OE for the outputs you want. If you need the Carry In function on pin 23 that will have its output disabled.

Depending which speed version you get, you can run at maximum clock frequencies of 80Mhz to 166MHz. The only possible downside is that it needs a 5V supply, where a 74HC, it it existed, would have been able to run off 3.3 if you'd needed it to.

The 16F873 has 28 pins so that can't occupy the same footprint, though it's not too much bigger. I'm pretty sure there aren't any 24 pin PICs.

\$\endgroup\$
6
  • \$\begingroup\$ Thanks for the Amtel recommendation. I'm using the gal16v8d's for address decoding. Fitting those extra pins in may be a killer. Its real tight. I'll sit down tonight and design up a 22V10 and sim it to see if it works. Jack KG6INX \$\endgroup\$ – forjack842 May 3 '17 at 0:09
  • \$\begingroup\$ The 22v may have a cmos components but the Icc sucks. Granted about half of the LS but at 65-100ma is still a battery killer x5 chips. \$\endgroup\$ – forjack842 May 3 '17 at 17:47
  • \$\begingroup\$ The typical value for the CQ-15 part clocked at 15MHz is 40mA. How much were you expecting your HC461s to take? \$\endgroup\$ – Finbarr May 4 '17 at 9:13
  • \$\begingroup\$ I found Cypress PALCE22VA10Z which gives me the best idle current while still being less than the others so far. Working logic equations. The 461s manage the addrs of both the data memory chips & processor memory providing high speed DMA transfers then going idle til again commanded. I am still amazed on the complete lack of anything like 461 in cmos. This chip is exactly made for this kind of app enhancing & off loading a 8051. My 1st design used 573s only to find massive overhead writing addr by addr & data bus I/O. The ls461 worked nice!..just no cmos version. The LS vers runs hot! \$\endgroup\$ – forjack842 May 5 '17 at 3:22
  • \$\begingroup\$ I think the fact that you're having to use five of them in your design probably explains why people would look to do this a different way. Some of the Cortex-based MCUs have over a dozen DMA channels built in nowadays. \$\endgroup\$ – Finbarr May 5 '17 at 8:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.