Page 99 - Embedded Microprocessor Systems Real World Design

a DMA transfer. The DMA controller requests the bus, does the transfer, and gives
the bus back to the processor. The DMA controllers are programmable and can
transfer data from memory to memory, from memory to I/O, from 1/0 to memory,
and from 1/0 to I/O. Most importantly, all the arbitration is handled inside the
microprocessor, which saves on hardware and ensures that all the timing is correct.
In addition, an internal DMA controller can usually transfer data to and from inter-
nal RAM (internal to the CPU IC), which an external DMA controller cannot do.
DMA requests can come from a source that needs to send data (such as a UART
that has received a byte and needs to transfer it to memory) or from a destination
device that needs to receive data (the same UART may have just transmitted a byte
and needs a new one). In some systems, it does not matter whether the DMA
requester is a source or destination device. In other cases it does.
The 80186/80188 DMA controllers support either source or destination
requests, which are referred to on the datasheet as source or destination synchro-
nization. In practice, either device can be the requester since source/destination
synchronization is programmed into the DMA controller by the firmware. However,
the choice of source versus destination synchronization affects the timing. Source
synchronization permits faster transfers but requires that the DMA request be
removed before the end of the DMA write cycle. Destination synchronization is
slower but has more relaxed timing for removal of the DMA request.
The Motorola MCF5407 ColdFire processor provides an internal four-channel
DMA controller. Two channels support DMA requests from external devices and
two permit the internal UARTs to transfer data to memory.
As mentioned earlier, DMA can be used for dynamic memory refresh. This is
accomplished by using a timer to request a periodic DMA cycle at the memory
refresh rate. The DMA controller is programmed to read the memory and write
the contents to a nonexistent memory or 1/0 location. The result is a periodic
read from memory that refreshes memory contents.

Watchdog Timers

A microprocessor executes instructions from memory. If a nearby electrical dis-
charge occurs, the processor data bus may be momentarily upset, and the proces-
sor can get a bad byte from the PROM. Or a software bug can result in a stack
overflow, and the processor gets garbage when it tries to return from a subroutine.
In either case, the processor will usually go “into the weeds,” which is a shorthand
way of saying that it will begin executing code in some unpredictable way, usually
resulting in a system crash.
If this happens to the keyboard controller in your PC, you can just turn the power
off and back on, and everything will be fine. If it happens to the processor that

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