Date: Sat, 20 Jul 1996 23:10:08 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, wong@wong.rogerswave.ca Cc: E00114@vnet.atea.be, freebsd-hardware@freebsd.org Subject: Re: Multiple COM ports with same IRQ Message-ID: <199607201310.XAA32484@godzilla.zeta.org.au>
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>> 16550 UARTs just generate signals that should work with either level or >> edge sensitive schemes. If the interrupts are still edge sensitive from >> the 8259 PIC's point of view, then they can't be shared. > ^^^^^^^^^ >I thought all 8259 PIC can be programmed to use either edge or level. >anyway at least mine said it does. If it is programmed for level triggered interrupts, then all devices have to generate IRQ signals that stay asserted while there is an IRQ. Some don't. I think at least MFM disk controllers usually assert IRQ and pulse it low to cause an edge. I think the 8254 clock in RATEGEN (standard FreeBSD) mode is similar. Perhaps more importantly, it must be possible to program the device to stop it interrupting. This is inconvenient for the 8254 clock. In SQWAVE (standard BIOS) mode it asserts IRQ half the time and there is no good way to stop it interrupting half the time if interrupts are level sensitive. Bruce
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