M16C/62 Group DATASHEET

DMACMitsubishi microcomputersM16C / 62 GroupSINGLE-CHIP 16-BIT CMOS MICROCOMPUTER(1) Transfer cycleThe transfer cycle consists of the bus cycle in which data is read from memory or from the SFR area(source read) and the bus cycle in which the data is written to memory or to the SFR area (destinationwrite). The number of read and write bus cycles depends on the source and destination addresses. Inmemory expansion mode and microprocessor mode, the number of read and write bus cycles also dependson the level of the BYTE pin. Also, the bus cycle itself is longer when software waits are inserted.(a) Effect of source and destination addressesWhen 16-bit data is transferred on a 16-bit data bus, and the source and destination both start at oddaddresses, there are one more source read cycle and destination write cycle than when the sourceand destination both start at even addresses.(b) Effect of BYTE pin levelWhen transferring 16-bit data over an 8-bit data bus (BYTE pin = “H”) in memory expansion mode andmicroprocessor mode, the 16 bits of data are sent in two 8-bit blocks. Therefore, two bus cycles arerequired for reading the data and two are required for writing the data. Also, in contrast to when theCPU accesses internal memory, when the DMAC accesses internal memory (internal ROM, internalRAM, and SFR), these areas are accessed using the data size selected by the BYTE pin.(c) Effect of software waitWhen the SFR area or a memory area with a software wait is accessed, the number of cycles isincreased for the wait by 1 bus cycle. The length of the cycle is determined by BCLK.Figure 1.16.5 shows the example of the transfer cycles for a source read. For convenience, the destinationwrite cycle is shown as one cycle and the source read cycles for the different conditions are shown.In reality, the destination write cycle is subject to the same conditions as the source read cycle, with thetransfer cycle changing accordingly. When calculating the transfer cycle, remember to apply the respectiveconditions to both the destination write cycle and the source read cycle. For example (2) in Figure1.16.5, if data is being transferred in 16-bit units on an 8-bit bus, two bus cycles are required for both thesource read cycle and the destination write cycle.77