參數(shù)資料
型號： MC68CK338
廠商： Motorola, Inc.
元件分類： 32位微控制器
英文描述： Highly Integrated, Low-Power, 32-Bit Microcontroller
中文描述： 高度集成，低功耗，32位微控制器
文件頁數(shù)： 26/133頁
文件大?。?/td> 798K
代理商： MC68CK338
MOTOROLA
26
MC68CK338
MC68CK338TS/D
PTP — Periodic Timer Prescaler Control
0 = Periodic timer clock not prescaled
1 = Periodic timer clock prescaled by 512
The reset state of PTP is the complement of the state of the MODCLK signal at the rising edge of
RESET.
PITM[7:0] — Periodic Interrupt Timer Modulus
This is an 8-bit timing modulus. The period of the timer can be calculated as follows:
where
PIT Period = Periodic interrupt timer period
PITM[7:0] = Periodic interrupt timer register modulus
f
ref
= Synthesizer reference of external clock input frequency
Prescaler = 1 if PTP = 0 or 512 if PTP = 1
3.5 External Bus Interface
The external bus interface (EBI) transfers information between the internal MCU bus and external de-
vices. The external bus has 24 address lines and 16 data lines.
The EBI provides dynamic sizing between 8-bit and 16-bit data accesses. It supports byte, word, and
long-word transfers. Ports are accessed through the use of asynchronous cycles controlled by the size
(SIZ1 and SIZ0) and data size acknowledge (DSACK1 and DSACK0) pins. Multiple bus cycles may be
required for dynamically sized transfer.
Port width is the maximum number of bits accepted or provided during a bus transfer. External devices
must follow the handshake protocol described below. Control signals indicate the beginning of the cycle,
the address space, the size of the transfer, and the type of cycle. The selected device controls the length
of the cycle. Strobe signals, one for the address bus and another for the data bus, indicate the validity
of an address and provide timing information for data. The EBI operates in an asynchronous mode for
any port width.
To add flexibility and minimize the necessity for external logic, MCU chip-select logic can be synchro-
nized with EBI transfers. Chip-select logic can also provide internally-generated bus control signals for
these accesses. Refer to
3.6 Chip-Selects
for more information.
3.5.1 Bus Control Signals
The CPU initiates a bus cycle by driving the address, size, function code, and read/write outputs. At the
beginning of the cycle, size signals SIZ0 and SIZ1 are driven along with the function code signals
(FC[2:0]). The size signals indicate the number of bytes remaining to be transferred during an operand
cycle. They are valid while the address strobe AS is asserted.
Table 13
shows SIZ0 and SIZ1 encoding. The read/write (R/W) signal determines the direction of the
transfer during a bus cycle. This signal changes state, when required, at the beginning of a bus cycle,
and is valid while AS is asserted. The R/W signal only changes state when a write cycle is preceded by
a read cycle or vice versa. The signal can remain low for two consecutive write cycles.
PIT Period
----------------------------------------------------------------
f
ref
)
=
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