z89136 ZiLOG Semiconductor, z89136 Datasheet - Page 35

• Manufacturer Part Number: z89136
• Description: Low-cost Dtad Controller
• Manufacturer: ZiLOG Semiconductor

Zilog
Counter/Timers. There are two 8-bit programmable
counter/timers (T0-T1), each driven by its own 6-bit pro-
grammable prescaler. The T1 prescaler is driven by inter-
nal or external clock sources; however, the T0 prescaler is
driven by the internal clock only (Figure 23).
The 6-bit prescalers can divide the input frequency of the
clock source by any integer number from 1 to 64. Each
prescaler drives its counter, which decrements the value
(1 to 256) that has been loaded into the counter. When the
counter reaches the end of the count, a timer interrupt re-
quest, IRQ4 (T0) or IRQ5 (T1), is generated.
The counters can be programmed to start, stop, restart to
continue, or restart from the initial value. The counters can
also be programmed to stop upon reaching zero (single
DS97TAD0300
(DSP CON)
D7, D6
(F) OC
DSP Clock
D0,D1
(SMR)
Clock
TIN P31
Logic
16
2
2
External Clock
Internal Clock
Gated Clock
Triggered Clock
Internal
Clock
4
Figure 23. Counter/Timer Block Diagram
2
T0, T2, T3
P R E L I M I N A R Y
OSC
4
Write
Write
Initial Value
Initial Value
Register
Register
Counter
Counter
PRE0
PRE1
Down
Down
6-Bit
6-Bit
pass mode) or to automatically reload the initial value and
continue counting (modulo-n continuous mode).
The counters, but not the prescalers, are read at any time
without disturbing their value or count mode. The clock
source for T1 is user-definable and is either the internal mi-
croprocessor clock divided by four, or an external signal in-
put through Port 31. The Timer Mode register configures
the external timer input (P31) as an external clock, a trig-
ger input that can be retriggerable or non-retriggerable, or
as a gate input for the internal clock. The counter/timers
can be cascaded by connecting the T0 output to the input
of T1.
Write
Write
Internal Data Bus
Internal Data Bus
Initial Value
Initial Value
Register
Register
Counter
Counter
Down
Down
8-Bit
8-bit
T0
T1
Read
Read
2
Current Value
Current Value
Register
Register
Low-Cost DTAD Controller
T0
T1
Z89135/136 (ROMless)
T
P36
IRQ5
IRQ4
OUT
1-35
1