Page 125 - Embedded Microprocessor Systems Real World Design

tents of Data Out into the “D” flip-flop. Only DO is used to transfer data; the other
bits are unused.
After writing a bit to the device, the CPU must read the contents of the flip
flop to avoid losing that bit. After all the bits are transferred, the chip select signal
is driven high to terminate the cycle. Note that reading the data bit from the
flip-flop does not generate a clock to the Microwire device; only write cycles do
that.
If you don’t want to read the data after each bit is written, you could replace the
“D” flip-flop with a shift register and read the entire register when the entire data
stream has been written. The length of the shift register must be the same as the
number of bits to be written/read from the device.
If you are writing to a device that is write-only, you would not need the “D” flip-
flop or the tristate buffer. Although this approach still requires that the CPU do
bit-by-bit transfers, it is somewhat faster than requiring the CPU to generate each
signal transition as would be required in a port-bit implementation.

Timer Basics

Timers are a crucial part of many embedded systems. Figure 3.5A shows a simple
timer like you might find in a microcontroller. This timer consists of a simple, load-
able %bit counter. You could build this from a couple of 74HC161 counters or
equivalent PLD logic.
The microprocessor can write a value to the timer that is transferred to the
counter outputs. If the counter is an UP counter (as shown), it counts up. A DOWN
counter counts down. A typical timer embedded in a microcontroller or in a timer
IC will have some means to start the timer once it is loaded, typically by setting a
bit in a register. The clock input to the counter may be a derivative of the micro-
processor clock or it may be a signal applied to one of the external pins. A real
timer will also provide the outputs of the counter to the microprocessor so it can
read the count, but for simplicity that has been left off Figure 3.5A.
If the microprocessor loads this timer with a value of OxFE and then starts the
timer, it will count from FE to FF on the next clock. On the second clock, it will
count from FF to 00 and generate an output. The output of the timer may set a
flip-flop that the microprocessor can read, or it may generate an interrupt to the
microprocessor, or both. The timer may stop once it has generated an output, or
it may continue counting from 00 back to FF. The problem with a continuously
running timer is that it will count from the loaded value the first time it counts up,
but the second time it will start from 00.

Hardware Design 2 107