PIOs & RRRepeat KBDs

A few newsletters back we did an article about the PIO (MK3881), and tried to get readers out of the ‘Catch 22’ situation, where the PIO technical manual could only be understood if the reader knew something about PIO’s. The article dealt with the general aspects of the PIO, and how to use it. Well Richard has been busy, and has sent us this:

Understanding the PIO

The PIO manual provides a good description of how this powerful device works, but there are some aspects of it which are not easy to understand. Perhaps the most confusing of all is the use of the ‘handshake’ signals, known as STROBE and READY lines. This article will briefly explain how input and output modes work, and then will give an example showing how two PIO’s can communicate, when one is the output and the other is the input.

Input mode

The PIO port must be set to mode 1 and interrupts enabled. As usual the I register, interrupt vector and interrupt mode must be set up correctly, and the PIO must not think that it is already processing an interrupt, so a dummy RETI instruction should be executed. Also, when in input mode, a dummy read should be issued once at the start to initialize handshaking. At last you are ready to enable the CPU interrupts with an EI instruction.

The sequence of events is as follows:

The input STROBE line goes from low to high, indicating that data is ready in the PIO input register. This change causes an interrupt, and the interrupt routine gets the data with an IN instruction. Issuing this instruction gets the data and also causes the PIO READY output line to go from low to high, indicating that the data has been received. The interrupt routine ends by enabling the CPU interrupts again and then immediately returns with a RETI, which re-enables the PIO interrupts.

Output mode

The PIO port must be prepared as for input mode, except that it is set to mode 0.

The sequence of events is as follows:

The input STROBE line goes from low to high, indicating that the previous output has been received. This change causes an interrupt, and the interrupt routine outputs the data with an OUT instruction. Issuing this instruction not only outputs the data to the PIO, but also causes the PIO READY line to go high, indicating that data is ready in the PIO. The interrupt