© 1995 Microchip Technology Inc.

DS00597A-page 3

AN597

transmit pulse. The instantaneous drain on the power

supply can be mitigated by installing a storage

capacitor across the power lines at the ranging module.

A value of 500 microfarads is recommended.

A 200 millisecond interval is recommended between

ranging cycles (Figure 2) to allow the transducer to

clear.

The ECHO line requires a pull-up resistor (4.7 k

Ω was

used in this application).

There must be a common ground between the

PIC16C74 circuitry and the ranging module.

Some applications may not need the resources of the

higher end PIC16CXX devices. It is still possible to do

this application using a device that does not contain a

CCP module (for ECHO timing). The capture function

can be implemented in firmware. The effect of a

firmware implementation is that the resolution of the

ECHO time would be 3 Tcy cycles versus 1 Tcy cycle

for

the

CCP

module.

Also,

the

firmware

implementation would not allow other tasks to be

performed while the capture function was occurring.

Refer to Appendix A for general ranging module

specifications.