TL16C752B DUART: MCR_OUT2 disables interrupts

TL16C752B DUART: MCR_OUT2 disables interrupts

[Shaun Jackman]

The datasheet I have for the TL16C752B labels the MCR_OUT1 bit as
`FIFO Rdy enable' and the MCR_OUT2 bit as `IRQ enable'. The latter bit
concerns me. The 8250.c driver sets MCR_OUT2 by default; however, if
the user space clears MCR_OUT2 (through an ioctl TIOCMSET operation or
similar), it seems to me that interrupts for that UART will stop
working. Can someone confirm my suspicion?

I'd expect that clearing/setting the OUT1 and OUT2 pins from user
space should be an innocuous operation. Disabling interrupts is a
fairly nasty side effect.

Please cc me in your reply. Cheers,
Shaun

Re: TL16C752B DUART: MCR_OUT2 disables interrupts

[Paul Fulghum]

Shaun Jackman wrote:
> The datasheet I have for the TL16C752B labels the MCR_OUT1 bit as
> `FIFO Rdy enable' and the MCR_OUT2 bit as `IRQ enable'. The latter bit
> concerns me. The 8250.c driver sets MCR_OUT2 by default; however, if
> the user space clears MCR_OUT2 (through an ioctl TIOCMSET operation or
> similar), it seems to me that interrupts for that UART will stop
> working. Can someone confirm my suspicion?
> 
> I'd expect that clearing/setting the OUT1 and OUT2 pins from user
> space should be an innocuous operation. Disabling interrupts is a
> fairly nasty side effect.

The old 16550 OUT2 output (MCR:3) used to be a truly
independant control output that was commonly used
with an external gate to connect/disconnect the
interrupt request to the ISA bus.

Reading the TL16C752B datasheet, it seems pretty clear
that this gating is performed internal to the device.
MCR:3 is no longer an independant, general purpose I/O.

The result is the same though: If you dink around with
MCR:3 you could disable interrupts in either case.

So I don't see this as more or less of a problem than
other 16550 type devices.

-- 
Paul Fulghum
Microgate Systems, Ltd.

Re: TL16C752B DUART: MCR_OUT2 disables interrupts

[Shaun Jackman]

On 6/20/06, Paul Fulghum <paulkf@microgate.com>
> The old 16550 OUT2 output (MCR:3) used to be a truly
> independant control output that was commonly used
> with an external gate to connect/disconnect the
> interrupt request to the ISA bus.
>
> Reading the TL16C752B datasheet, it seems pretty clear
> that this gating is performed internal to the device.
> MCR:3 is no longer an independant, general purpose I/O.
>
> The result is the same though: If you dink around with
> MCR:3 you could disable interrupts in either case.
>
> So I don't see this as more or less of a problem than
> other 16550 type devices.

I didn't know of this historic use of the OUT2 pin to disable
interrupts. I'll put OUT2 on my list of things not to muck with. Is
there any similar historic use of OUT1?

Cheers,
Shaun

Re: TL16C752B DUART: MCR_OUT2 disables interrupts

[Paul Fulghum]

Shaun Jackman wrote:
> On 6/20/06, Paul Fulghum <paulkf@microgate.com>
> I didn't know of this historic use of the OUT2 pin to disable
> interrupts. I'll put OUT2 on my list of things not to muck with. Is
> there any similar historic use of OUT1?

Not that I'm aware of. All of the old designs I've
seen left that signal unconnected.

-- 
Paul Fulghum
Microgate Systems, Ltd.