Title: DFlex UART IRQs cannot be used when loopback mode is turned on
Keywords: DFlex, UART, IRQ, Loopback
Date: March 26, 2002
On the DFlex board the operation of the IRQ is intimately involved with the OP2 pin of the UARTchip. This pin is controlled by Bit-3 of the MCR register.
The OP2 pin of each UARTenables its respective interrupt, when driven low (on), (MCR-bit3=1). When loopback mode is enabled the OP2 pin becomes “disconnected” from the OP2 register bit causing the OP2 pin to go high (off). This effectively disables the interrupt from that UART.
If all UARTs are put into loopback mode simultaneously, then there is one other effect… the ISA bus IRQ signal is tri-stated. When the signal is tri-stated the pull-up resistor on the IRQ signal will cause the IRQ signal to go high. The pull-up resistor is usually implemented the PC mother-board. The same effect can be created by setting all the UARTs MCR bit-3’s to zero.
So the net effect is that the UARTcannot be used with its interrupt when loopback mode is turned On. This behaviour can only be changed by changing the logic of the GAL.
History: The relationship of the OP2 pin and the Interrupt is a legacy issue with early XT and AT bus PCs. It’s a mechanism which allowed the allocation/sharing of interrupts with mother board devices. The DFlex is a product which was used with these early computers.
End of KDB-282