> From:         Bryan Mumford[SMTP:bryan@BMUMFORD.COM]
> Sent:         Wednesday, December 17, 2003 12:58 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC:] 16F627 won't pull high

> It's maddening. Such a simple project. I have a 16F627 supposed to
> drive some transistors with a pulse. The scope shows the port B
> outputs are low at rest, as they are supposed to be. But when they
> are commanded to pull high, they seem to go tri-state instead. The
> scope shows the lines float, with 60 cycle noise, for the duration of
> the supposed high state (.01 seconds). Then they return to a low
> state.

> How can this be? The TRIS register is not changed. They output a low,
> but won't pull high. Even a spare bit, unconnected to anything,
> behaves the same way.

> The clockout pin shows the internal oscillator is running, and the
> circuit works fine when emulated with the Mathias ICE. I've tried
> several chips.

This behavior is typical of open drain transistor outputs. Check the
date sheet to see if those port pins are open drain. Also, try pullup
resistors on the outputs, and see what happens.

John Power

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