ZiLOG Z8

Z8 Microcontrollers
Counter/Timers ZiLOG
6-4 UM001601-0803
6.3.2 Prescaler Operations
During counting, the programmed clock source drives the 6-bit
Prescaler Counter. The counter is counted down from the value
specified by bits of the corresponding Prescaler Register, PRE0
(bit 7 to bit 2) or PRE1 (bit 7 to bit 2). (Figures 6-3, 6-4). When
the Prescaler Counter reaches its end-of-count, the initial value
is reloaded and counting continues. The prescaler never actually
reaches 0. For example, if the prescaler is set to divide-by-three,
the count sequence is:
Each time the prescaler reaches its end of count a carry is gener-
ated, that allows the Counter/Timer to decrement by one on the
next timer clock input. When the Counter/Timer and the prescal
-
er both reach the end-of-count, an interrupt request is generated
(IRQ4 for T0, IRQ5 for T1). Depending on the counting mode
selected, the Counter/Timer will either come to rest with its val
-
ue at 00H (Single-Pass Mode) or the initial value will be auto-
matically reloaded and counting will continue (Continuous
Mode). The counting modes are controlled by bit 0 of PRE0 and
bit 0 of PRE1. (Figure 6-8). A 0, written to this bit configures the
counter for Single-pass counting mode, while a 1 written to this
bit configures the counter for Continuous mode.
The Counter/Timer can be stopped at any time by setting the En-
able Count bit to 0, and restarted by setting it back to 1. The
Counter/Timer will continue its count value at the time it was
stopped. The current value in the Counter/Timer can be read at
any time without affecting the counting operation.
Note: The prescaler registers are write-only and cannot be read.
New initial values can be written to the prescaler or the
Counter/Timer registers at any time. These values will be trans
-
ferred to their respective down counters on the next load opera-
tion. If the Counter/Timer mode is continuous, the next load oc-
curs on the timer clock following an end-of-count. New initial
values should be written before the desired load operation, since
the prescalers always effectively operate in Continuous count
mode.
The time interval (i) until end-of-count, is given by the equation:
The internal clock frequency defaults to the external clock
source (XTAL, ceramic resonator, and others) divided by 2.
Some Z8 microcontrollers allow this divisor to be changed via
the Stop-Mode Recovery register. See the product data sheet for
available clock divisor options.
Note that t is equal to eight divided-by-XTAL frequency of the
external clock source for T1 (external clock mode only).
p = the prescaler value (1 – 63) for T
0
and T
1
.
The minimum prescaler count of 1 is achieved by loading
000001xx. The maximum prescaler count of 63 is achieved by
loading 111111xx.
v = the Counter/Timer value (1-256)
Minimum duration is achieved by loading 01H (1 prescaler out-
put count), maximum duration is achieved by loading 00H (256
prescaler outputs counts).
The prescaler and counter/timer are true divide-by-n counters.
3–2–1–3–2–1–3–2–1–3...
i = t X p X v
in which:
t = four times the internal clock period.