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Functional Description
Interval Timers
6
Serial Port Signal Multiplexing
6-16
The PBC has three built-in counters that are equivalent to those found in
an 82C54 programmable interval timer. The counters are grouped into one
timer unit, Timer 1, in the PBC. Each counter output has a specific
function:
Counter 0 is associated with interrupt request line IRQ0. It can be
used for system timing functions, such as a timer interrupt for a
time-of-day function.
Counter 1 generates a refresh request signal for ISA memory. This
timer is not used in the MCPN750A.
Counter 2 provides the tone for the speaker output function on the
PBC (this timer output is not used in the MCPN750A).
The interval timers use the OSC clock input as their clock source. The
MCPN750A drives the OSC pin with a 14.31818 MHz clock source.
Due to pin limitations of the J3 connector, the MCPN750A multiplexes
and de-multiplexes some signals between the MCPN750A board and the
TMCPN710 and the TM-PIMC-0001 transition modules. This hardware
function is transparent to the software. The block diagram for the signal
multiplexing is shown in the following figure:
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