Hi everyone,
I wonder if anyone here is familiar with the Holtek range of encoders and in particular the HT12A?
Please see the datasheet for the device in my attachment. My confusion surrounds the transmit enable part of the device. According to the datasheet D8-D11 are used as the data inputs and transmit enable pins simultaneously. They are active low, so the device will transmit data when one of these pins is driven low. If a data pin is driven low briefly it will transmit one complete cycle. If a data pin is held low then it will transmit continuously. I've obtained this information from the pin description on page 3 and the transmission timing diagrams on page 5.
Now, here is where my confusion starts. The pin description on page 3 says that the data pins D8-D11 should be externally set to VSS or left open. This appears to contradict the transmit timing of the device, which shows that if a data pin is held low it will transmit continuously. If I don't want to transmit continuously, then surely I shouldn't be externally setting any of those pins to VSS?
The second part of my confusion surrounds data transmission. The only way to trigger the device is to set one of the data pins low. But what if I wanted to transmit 1111? How would I then trigger the device? Also, what if I only want to briefly transmit a nibble of data? I would have to set the data nibble on pins D8-D11 for a short time and then immediately tie them to 1111 afterwards to stop it transmitting. It all seems a little contradictory and silly?
Am I not understanding fully how this device is supposed to operate? My feeling is that the device is severely limited by using the data pins themselves as transmit enable pins. There should infact be a seperate transmit enable pin, as for the HT12E. Unfortunately I want to use the 38Khz carrier that is a feature of the HT12A so it's not simply a case of my choosing to use the HT12E instead.
If anyone can clear up my confusion, or agree with my conclusions then I would appreciate your response.
Brian