Singapura
Member level 3
TTL Problems
I have a PCB which I am presently troubleshooting. The portion which I
encountered some strange phenomenon is depicted in the attached
diagram.
Here, a free running oscillator clocks a counter which drives the address
of an EPROM. The data output of the EPROM is then latched to drive the
input of a DAC which then outputs a 19.2KHz sine wave. Except for the
DAC which operates from +15Vdc and -15Vdc, the rest of the circuit is
basically TTL.
In the process of monitoring the signals of this circuit, I discovered that
most of the time, the EPROM data output does not produce TTL-level
data activities. Instead, random spikes of approximately 2V were seen
and these spikes were latched to the DAC, causing the DAC to output
nothing.
The question is, for a digital device such as an EPROM, how is it possible
that random spikes can be generated at its output when it is directly
driving a latch only? Occassionally, I do obtain the desired data activities
expected but once I power off and on again, the random spikes reappear
again. What could have caused it?
Can anyone who've experienced this before give me some help?
Thanks in advance.
I have a PCB which I am presently troubleshooting. The portion which I
encountered some strange phenomenon is depicted in the attached
diagram.
Here, a free running oscillator clocks a counter which drives the address
of an EPROM. The data output of the EPROM is then latched to drive the
input of a DAC which then outputs a 19.2KHz sine wave. Except for the
DAC which operates from +15Vdc and -15Vdc, the rest of the circuit is
basically TTL.
In the process of monitoring the signals of this circuit, I discovered that
most of the time, the EPROM data output does not produce TTL-level
data activities. Instead, random spikes of approximately 2V were seen
and these spikes were latched to the DAC, causing the DAC to output
nothing.
The question is, for a digital device such as an EPROM, how is it possible
that random spikes can be generated at its output when it is directly
driving a latch only? Occassionally, I do obtain the desired data activities
expected but once I power off and on again, the random spikes reappear
again. What could have caused it?
Can anyone who've experienced this before give me some help?
Thanks in advance.