Parallel display interface- 4 bit vs 8 bit or serial?

Don M

I have an application using a VFD display. The display needs to be updated or refreshed rather quickly showing a number counting.
This is probably a silly question but would an 8 bit interface update quicker than a 4 bit? Would it be twice as fast? Right now I compare it to using PST which updates real quick. Is it possible to achieve speeds comparable to using PST?
How about a serial interface?

Sorry for all the questions...

Thanks.

Don

Mike Green

It probably doesn't matter. A visual display can usually be updated faster than you can see the changes regardless of whether you're using a serial interface or a 4-bit or 8-bit interface. It depends on the amount of data needed. Anything more than around 60 refreshes a second is fast enough. There have been experiments on subliminal perception that have shown some responses on visual data down to a few milliseconds, but that's a special case.

An 8-bit interface can transfer roughly twice as much data in the same time as a 4-bit interface. There's some additional overhead likely in transferring 8-bit data via a 4-bit interface, but you're talking about digits which would fit into 4 bits naturally if that's the way the specific 4-bit interface would work. PST uses 8-bit serial data (actually 10 bits if you include the start and stop bits).

StefanL38

So this means it depends more on the baudrate of the display's interface.
If you can transfer at 115200 baud on a serial link this means 115200 / 10 bits = 11520 byte per second.
Refreshing 60 times a sceond means that you can transfer 11520 / 60 = 192 bytes at this refreshrate.

How fast do the digits count up?

If you want more specific help please provide a working downloadlink to the datasheet of the displays interface.
(What kind of interface does it have HD44780-controller like LCDs is it similar to 7-segment.display?
keep the questions coming
best regards

Stefan

Don M

The digits count up every 45 mS or about 22 - 23 times a second. I have attached the data info for the VFD display and controller chip. It appears to be HD44780 compatible as it works with various objects based on that chip.

StefanL38

to do the rest of the math 192 bytes per second / 23 refreshs per second means 8 bytes can be refreshed at that refershing-rate.

counting up 23 times per second can't be followed by a human beeing. Do you watch the display with a high-speed slow-motion camera that really every single digit-count has to be shown?
Indeed for car-chrash-tests this could be useful.

From what I saw in the datasheets the controller has a motorola M68-interface (parallel 8 bit, an intel I80-interface parallel 8 bit and a serial interface similar to SPI but to me it seems not HD44780-conform.
You wrote

It appears to be HD44780 compatible as it works with various objects based on that chip.

Does this mean you have already tested the display with HD44780-objects for the propeller?

So now I know what is the refreshing-rate. But a 23 times per second counting display is not a selfpurpose. What is it showing?
- microns on a CNC-maschine?
- revolutions of a cable-spindle?

keep the questions coming
best regards

Stefan

Don M

My inference to the HD44780 compatibility is from trying different objects from the OBEX that mention this compatibility and they all seem to work with my VFD display thus far.

As far as seeing the digits count up one by one- no I can't see them individually but am looking at the comparison of how they appear to count between seeing them on my monitor using PST and on the actual VFD display itself.

Since my last response I have tried another driver from the OBEX - http://obex.parallax.com/objects/667/ in particular the LCD_Nx2 and LCDBase_4Bit_direct_HD44780 objects and they appear to refresh the VFD display much faster but I am still in the middle of testing. My previous object that I was using was this- LCD_16x2_4Bit by Simon Ampleman. That one seems to refresh slow in my testing.

I'll update after further testing.

Don M

After further testing I'm happy with using this http://obex.parallax.com/objects/667/ it refreshes the screen as fast as the PST does on my monitor. So the answer to the original question I had about which interface to use I have found out that for my application the 4 bit interface is fast enough.

Thanks everyone for your help and comments.

Don