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The PLL cannot be switched off and on while the EM1001 is running. This is because
when the PLL mode changes its output needs some time to stabilize. For this
reason, the PLL mode of the EM1001 can only be changed on reset. A special
internal delay circuit will hold the EM1001 in reset while the PLL frequency stabilizes.
The state of the PM pin at power-up or after the external reset (i.e. reset pulse on
the RST line) defines whether the EM1001 will run with PLL on or off. To have the
PLL on, leave the PM pin unconnected. To disable PLL and run at lower clock
frequency ground the PM pin prior to turning on the power or resetting the board.
Your Tibbo BASIC/C application can also change the PLL mode programmatically.
The Tibbo BASIC/C application can check the current PLL mode through the system
(sys.) object (see TIDE, TiOS, Tibbo BASIC, and Tibbo C Manual). If the PLL mode
needs to be changed the application must set the desired new mode and then
perform an internal reset (again, through the system object). The internal reset is
identical to the power-on or external reset with one crucial difference: the PLL
mode is set basing on the mode requested by the application, while the PM line
state is disregarded.
MD button and line
The function of the MD button/line is described in Setup Button (MD line). The
EM1001 board has both the button and the MD line. The button is wired in parallel
with the MD line. Use an open collector circuit to control the MD line as
pressing the MD button short-circuits the line onto the GND.
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