HD44780 20ns rise time requirement on E pin

[pekfos]

I stumbled upon interesting requirement in HD44780 datasheet. On E (enable) pin they specify maximum rise and fall time of 20ns (in one datasheet, I found 25ns in another). For example here: https://www.sparkfun.com/datasheets/LCD/HD44780.pdf . I'm driving it from ATTiny2313 on breadboard with 2 inch jumper wire, and even measuring rise from max low state (0.6V) to min high state (2.2V) I get rise time of 44ns. This is not a high bandwidth protocol, why such tight requirement?

[KrudyZ]

I very much doubt that your actual rise time from the ATtiny is 44 ns.
What scope did you use to measure this with and what probing did you use?
Edit: Sorry I just saw your screen shot. Still what probing did you use for this...
The source current of an I/O pin on the ATTiny2323 is around 40 mA per data sheet for a 5V supply. Are you sure you have this set as a push-pull drive, not an open drain?
What is your fall time on this signal?

[T3sl4co1l]

Drive the pin high, don't use an open-drain method.

The requirement implies it's driving sequential logic i.e. flip-flops, counters, registers (type D, not merely latched), that sort of thing.

I don't know what the internal logic on those devices actually is, but it's certainly not just an IO register (latched), and the timing requirements imply something more like a state machine doing sequential processing (i.e. counting through memory addresses, etc.), versus something more general-purpose (like a mask-programmed MCU), or something more direct and high performance (like interleaved access or dual-port RAM).

There's also numerous clones of them (often with improvements like SPI, larger display support, more CGRAM etc., I don't know?) so there may be different reasons each variety uses those timing restrictions for.

Tim

[pekfos]

Seems like in the troubleshooting mess I wound up with 1k resistor in series with my probe . Now it's 21ns, but now it's time for full swing from 0 to 5V. Same for falling edge. Now measured directly on display module pins, using spring contact for ground. Rising transition through 0.6-2.2V region takes 6.8ns. If these numbers sound correct, then I guess problem was between scope and chair.

[T3sl4co1l]

Your probe is a whopping 40pF?!

Tim

[pekfos]

It was set to 1X 

[T3sl4co1l]

Oh well check the risetime of that mode too.  Even with a 50 ohm source you will find it's quite low (usually around 6MHz).

10x is better both because it gives full bandwidth, and loads the signal source less.

Tim

[pekfos]

With 10X settings full rise time measures 8ns. Probe has 50 +-20pF in 1X and 10 +- 5pF in 10X. 8ns is well within spec so problem solved.