Hi,
I'm playing with an Atmel 91SAM7S64. It has 5volt tolerant I/O. I've defined an input port, 8bits and print the sampled value as ASCII over a UART every 1/2second.
When I place a buffer 74HCT245N with it configured to drive the input lines, it draws like 180mA on reset and makes the micro freak out.
I should say, it only does this when driven by logic on the input side of the buffer, if I simply load the input side of the 74HCT245N with pullup's and dips to ground, it works.
If I remove the buffer, connecting the bus A-B then it works as expected, both driven with I/O and using the resistors and dips as mentioned above.
I want the buffer there as added input protection because the micro doesn't have diodes on the input lines.
So my question, why might I be getting hugh 180mA drains on the 5volts going to the buffer and why only when 5volt logic is driving its inputs.
Should its outputs to the micro have a series resistors (100R)? in most buses I've seen they don't.
Output enable for the buffer is software controlled and the problem at reset only occurs after I enable the buffer. I have double checked this and the direction control line with a scope.
Funny thing is, the 3volt supply doesn't seem to be affected but during the high current drain the ground is going crazy, jogging around at near a volt and ofcourse the 3.3volt and 5volt grounds are connected.
I notice, if I start the micro with the 5volts off, then once the micro is running, connect the 5volts it all seems to run. I should also say, while it runs and the scope shows the logic level at the micro pins change, they aren't read correctly by the micro.
Again, if I disconnect the buffer, wire link the buses, it reads and works as expected.
Circuit:
**broken link removed**
Any help greatly appreciated,
I'm playing with an Atmel 91SAM7S64. It has 5volt tolerant I/O. I've defined an input port, 8bits and print the sampled value as ASCII over a UART every 1/2second.
When I place a buffer 74HCT245N with it configured to drive the input lines, it draws like 180mA on reset and makes the micro freak out.
I should say, it only does this when driven by logic on the input side of the buffer, if I simply load the input side of the 74HCT245N with pullup's and dips to ground, it works.
If I remove the buffer, connecting the bus A-B then it works as expected, both driven with I/O and using the resistors and dips as mentioned above.
I want the buffer there as added input protection because the micro doesn't have diodes on the input lines.
So my question, why might I be getting hugh 180mA drains on the 5volts going to the buffer and why only when 5volt logic is driving its inputs.
Should its outputs to the micro have a series resistors (100R)? in most buses I've seen they don't.
Output enable for the buffer is software controlled and the problem at reset only occurs after I enable the buffer. I have double checked this and the direction control line with a scope.
Funny thing is, the 3volt supply doesn't seem to be affected but during the high current drain the ground is going crazy, jogging around at near a volt and ofcourse the 3.3volt and 5volt grounds are connected.
I notice, if I start the micro with the 5volts off, then once the micro is running, connect the 5volts it all seems to run. I should also say, while it runs and the scope shows the logic level at the micro pins change, they aren't read correctly by the micro.
Again, if I disconnect the buffer, wire link the buses, it reads and works as expected.
Circuit:
**broken link removed**
Any help greatly appreciated,