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參數(shù)資料
| 型號： | M68HC16Y1CFC |
| 廠商： | MOTOROLA INC |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | 16-BIT, MROM, MICROCONTROLLER, PQFP16 |
| 封裝： | QFP-160 |
| 文件頁數(shù)： | 83/138頁 |
| 文件大?。?/td> | 784K |
| 代理商： | M68HC16Y1CFC |
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MC68HC16Y1
MOTOROLA
MC68HC16Y1TS/D
49
3.5.7 Bus Cycle Termination Signals
During bus cycles, external devices assert the data transfer and size acknowledge signals (DSACK1
and DSACK0). During a read cycle, the signals tell the MCU to terminate the bus cycle and to latch data.
During a write cycle, the signals indicate that an external device has successfully stored data and that
the cycle may terminate. These signals also indicate to the MCU the size of the port for the bus cycle
just completed. (Refer to the discussion of dynamic bus sizing.)
The bus error (BERR) signal is also a bus cycle termination indicator and can be used in the absence
of DSACK1 and DSACK0 to indicate a bus error condition. It can also be asserted in conjunction with
these signals, provided it meets the appropriate timing requirements. The internal bus monitor can be
used to generate the BERR signal for internal and internal-to-external transfers. When BERR and HALT
are asserted simultaneously, the CPU16 takes a bus error exception.
Finally, autovector signal (AVEC) can be used to terminate external IRQ pin interrupt acknowledge cy-
cles. AVEC indicates that the MCU will internally generate a vector number to locate an interrupt handler
routine. If it is continuously asserted, autovectors will be generated for all external interrupt requests.
AVEC is ignored during all other bus cycles.
3.5.8 Data Transfer Mechanism
The MCU architecture supports byte, word, and long-word operands, allowing access to 8- and 16-bit
data ports through the use of asynchronous cycles controlled by the data transfer and size acknowledge
inputs (DSACK1and DSACK0).
3.5.9 Dynamic Bus Sizing
The MCU dynamically interprets the port size of the addressed device during each bus cycle, allowing
operand transfers to or from 8- and 16-bit ports. During an operand transfer cycle, the slave device sig-
nals its port size and indicates completion of the bus cycle to the MCU through the use of the DSACK0
and DSACK1 inputs, as shown in the following table.
For example, if the MCU is executing an instruction that reads a long-word operand from a 16-bit port,
the MCU latches the 16 bits of valid data and then runs another bus cycle to obtain the other 16 bits.
The operation for an 8-bit port is similar, but requires four read cycles. The addressed device uses the
DSACK0 and DSACK1 signals to indicate the port width. For instance, a 16-bit device always returns
DSACK0 for a 16-bit port (regardless of whether the bus cycle is a byte or word operation).
Dynamic bus sizing requires that the portion of the data bus used for a transfer to or from a particular
port size be fixed. A 16-bit port must reside on data bus bits [15:0], and an 8-bit port must reside on data
bus bits [15:8]. This minimizes the number of bus cycles needed to transfer data and ensures that the
MCU transfers valid data.
The MCU always attempts to transfer the maximum amount of data on all bus cycles. For a word oper-
ation, it is assumed that the port is 16 bits wide when the bus cycle begins. Operand bytes are desig-
nated as shown in the figure below. OP0 is the most significant byte of a long-word operand, and OP3
is the least significant byte. The two bytes of a word-length operand are OP0 (most significant) and OP1.
The single byte of a byte-length operand is OP0.
Table 15 Effect of DSACK Signals
DSACK1
DSACK0
Result
1
Insert Wait States in Current Bus Cycle
1
0
Complete Cycle — Data Bus Port Size is 8 Bits
0
1
Complete Cycle — Data Bus Port Size is 16 Bits
0
Reserved
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