Re: TL16C752B DUART: MCR_OUT2 disables interrupts

[Paul Fulghum <paulkf@microgate.com>]

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.