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Z80 Datasheet(PDF) 24 Page - Zilog, Inc. |
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Z80 Datasheet(HTML) 24 Page - Zilog, Inc. |
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24 / 330 page  <%27 2GTKRJGTCNU 7UGT /CPWCN UM008101-0601 Counter/Timer Channels the down-counter counts to zero, the down-counter is automatically reloaded with the contents of the Time Constant register. This process divides the System clock by an additional factor of the time constant. Each time the down-counter counts to zero, its output, Zero Count/Timeout (ZC/ TO), is pulsed High. The Time Constant Register The 8-bit Time Constant register is used in both Counter and Timer modes. It is programmed by the CPU just after the channel control word, with an integer time constant value of 1 through 256. This register loads the programmed value to the down-counter when the CTC is first initialized and reloads the same value into the down-counter automatically whenever it counts down thereafter to zero. If a new time constant is loaded into the Time Constant register while a channel is counting or timing, the present down count is completed before the new time constant is loaded into the down counter. For details about writing a time constant to a CTC channel, see “CTC Programming” on page 18 The Down-Counter The down-counter is an 8-bit register that is used in both COUNTER and TIMER modes. This register is loaded by the Time Constant register both initially, and when it counts down to zero. In the COUNTER mode, the down-counter is decremented by each external clock edge. In the TIMER mode, it is decremented by the clock output of the prescaler. By performing a simple I/O Read at the port address assigned to the selected CTC channel, the CPU can access the contents of the down-counter and obtain the number of counts-to-zero. Any of the four CTC channels may be programmed to generate an interrupt request sequence each time the zero count is reached. In Channels 0, 1, and 2, a signal pulse appears at the corresponding ZC/TO pin when the zero count condition is reached. Because of package pin limitations, however, Channel 3 does not have this pin and so may be used only in applications where this output pulse is not required. |
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