In SX Microcontrollers, SX/B Compiler and SX-Key Tool, aemileski wrote:

An attempt to clarify what I mean by "refresh":  cause an LED to display a given state.

This requires that:
1) Power to the LED must be on.
2) The LED state is latched.

Latching the LED state can be done at a much higher rate than turning the power OFF and ON*  This makes it possible to turn the power ON and latch different PWM state for the LED while leaving the power ON.

Ideally only turn the power OFF when leaving it ON would result in a LED displaying the wrong state, which only happens in a device with multiple rows where only one row can be powered at a time.

Most 74HC parts are guaranteed to operate at 30 MHz (33 ns), and have a max limit of 50 MHz (20 ns).  The actual limit really depends on the manufacturer, and where in the silicon wafer the chip was cut from, hence the wide range.



* This largely has to do with the time it takes for the voltage to ramp up because of stray capacitance on the board, as well as the response time of the power supply to a sudden increased load.  Some boards also have RC circuits to limit the ON time.

---------- End of Message ----------


You can view the post on-line at:
http://forums.parallax.com/forums/default.aspx?f=7&p=1&m=301606#m306083

Need assistance? Send an email to the Forum Administrator at forumadmin@parallax.com

The Parallax Forums are powered by dotNetBB Forums, copyright 2002-2008 (http://www.dotNetBB.com)