Holtek HT-648L Decoder

[ACDC]

This is my first venture into using a remote system consisting of a Holtek Ht-640 encoder and Ht-648L decoder, and I have questions, hopefully some one can help. The datasheets don’t have much in the way of application. <p>My project consists of the remote system, a PIC16F876 and two LMD18200 H-bridges, driving two 24V gear motors.<p>The encoder drives a transmitter (418 mhz) and the decoder has a receiver. The states of eight lines on the encoder (high/low 5v) show up on the decoder after you press transmit. <p>Can the decoder outputs be directly connected to the I/O lines on a PIC, is it that simple or am I missing something? <p>Are their some basic rules to follow in terms of parts layout, considering I have a receiver that will be close to a PIC and two H-bridges, driven by PWM? I know RF design can get exotic, but this is just a simple remote project to push buttons across the room.<p>Thanks in advance.

[rshayes]

The I/O lines on a PIC seem to be a CMOS input with TTL thresholds when the PIC is operated at 5 volts. The leakage currents are given as being less than 1 microamp.<p>The data sheets for the Holtek parts are not on their web site. Other similar parts seem to have CMOS output stages that have low current capability (about .25 milliamp) but will reach .5 and 4.5 volt thresholds when operated on 5 volts.<p>If both chips are operated on 5 volts, a direct connection should work.

[rshayes]

The PIC chips are not particularly high speed, so harmonics in the 400 MHz region will probably not be very strong. The early TTL series could trash the low channels on a nearby TV set, but the high VHF channels, at around 200 MHz, were much less affected.<p>Using the minimum clock rate that you can on the PIC and arranging for some physical separation may be good enough.<p>Try to keep the 24 volt equip isolated from the processor. If the motors are of the brush type, they can generate a fair amount of electrical noise. Try to keep the power supply and ground paths separated from the processor ground and power. Laying the circuits out with separate ground planes and then connecting the ground planes at one point (where the signals also pass) will help. The idea is to keep motor current from flowing across the ground plane of the other circuits. It is also a good idea to keep the ground currents of the logic circuits out of the ground and power planes of the receiver circuits.<p>Local power supply bypassing for each chip (.1 uF from each supply voltage to ground at each chip) and larger filter capacitors (10 to 100 uF electrolytic capacitors) for each section (receiver, logic, and motor drive) will also help. If the receiver and logic power supply currents are low, a small amount of series resistance (10 to 100 ohms) in series with the supply lead will make the electrolytic filter capacitor more effective.

[ACDC]

Both devices operate at 5 vdc, the Holtek devices has cmos in/out/puts. I found the datasheets at [url=http://www.rentron.com,]http://www.rentron.com,[/url] which is also where I
purchased the remote control kit.<p>Rentron seems to have one of the better PicBasic sites around along with the goodies they sell.<p>Using 4 Mhz resonators and have .1 uF caps on each power input, plus the 18200 H-Bridge data sheet recommends 100uF per amp of motor draw across power and ground. <p>Appreciate your input and will incorporate your suggestions into the pcb, once I get things proofed and running on the proto board.<p>Hopefully I can stay awake long enough tonight to test out the remote trans and rec circuits.