Z80185 Datasheet, PDF (87/95 Pages) Zilog, Inc. – SMART PERIPHERAL CONTROLLERS

Zilog
PRELIMINARY
Z80185/Z80195
SMART PERIPHERAL CONTROLLES
Z80185 PIA AND MISCELLANEOUS REGISTERS (Continued)
CTC Control Registers
Channel Control Byte
This byte is used to set the operating modes and param-
eters. Bit D0 must be a 1 to indicate that this is a Control
Byte (Figure 82).
The Channel Control Byte register has the following fields:
Bit D7. Interrupt Enable. This bit enables the interrupt logic
so that an internal INT is generated at zero count. Interrupts
are programmed in either mode, and may be enabled or
disabled at any time.
Bit D6. Mode Bit. This bit, along with bit 3, is used to select
either Timer mode or Counter mode (Table 8).
Bit D5. Prescaler Factor. This bit selects the prescaler
factor for use in the timer mode. Either divide-by-16 or
divide-by-256 is available.
Bit D4. Clock/Trigger Edge Selector. This bit selects the
active edge of the CLK/TRG input pulses.
Bit D3. Mode Bit. This bit, along with bit 6, selects either
Timer mode or Counter mode (Table 8).
Bit D2. Time Constant. This bit indicates that the next byte
programmed is time constant data for the downcounter.
CCW6
0
0
1
1
Table 8. CTC Operation Modes
CCW3 Operation
0 (Auto Start) Timer mode. The
prescaler is clocked by PHI, and the
counter is clocked by the prescaler.
Counting is enabled when the timer
constant is loaded.
1 Timer with CLK/TRG Trigger. The
prescaler is clocked by PHI, and the
counter is clocked by the prescaler.
Timing starts when the transition
specified by D4 is detected on the
PIA pin, or for CTC3-1, the ZC/T0
output of CTC2-0, respectively.
0 Classic Counter mode. The counter
is clocked by the PIA pin, or for
CTC3-1 the ZC/TO output of
CTC-2 respectively.
1 Long Counter mode. The prescaler
is clocked by the PIA pin, or for
CTC3-1 the ZC/TO output of
CTC-2, respectively, and the
counter is clocked by the prescaler.
Bit D1. Software Reset. Writing a 1 to this bit indicates a
software reset operation, which stops counting activities
until another time constant word is written.
DS971850301
87