Page 103 - Embedded Microprocessor Systems Real World Design

asynchronous serial input or a three-wire synchronous serial interface for in-circuit
programming. Either way, you must do something similar to the PIC arrangement
if these functions are used on the board in normal operation.

Internal Peripherals

A number of processors and microcontrollers have built-in peripherals. These
usually are the peripherals commonly used in many embedded designs. Nearly
every processor intended specifically for embedded systems includes at least one
timer. Other common peripherals include serial ports, DMA controllers, watchdog
timers, and interrupt control functions. Be sure you take into account specific
restrictions of the peripheral device in the microprocessor you are using.
As an example, the Atmel AT9OS8515 processor includes an 8-bit and a lGbit
timer. Both timers include a prescaler that can divide the input signal by 8,64,256,
or 1024. Both timers can be clocked from the internal processor clock or an exter-
nal pin. When using the external clock input, the signal is internally synchronized
with the CPU clock. Thus, the maximum frequency you can input to these timers
is about half the CPU clock. If you are using an 8MHz 9088515, the maximum
timer frequency is a little less than 4MHz. If you’re using an 8MHz 90S8515 but
running it at GMHz, the maximum timer input frequency is about 3 MHz.
Contrast that with the Microchip PIC16C62. The PIC16C62 also has timers that
can operate from an external clock. The internal clock on the PIC devices is one
quarter the external input, so it would appear that the PIC clock rate (20 MHz in/
4MHz CPU) limits you to a slower input clock rate than the fastest Atmel AT9OS
clock rate (8 MHz) . If the external clock input is synchronized to the CPU clock,
that is true. However, the PIC devices have a mode in which the timer clock is not
synchronized to the internal CPU clock. In this mode, the clock input frequency
can be as high as 50 MHz. However, in this mode, the timer cannot be used for any
operations that require synchronization, such as PWM or capture/compare.
Most embedded processors include pins that can be used as external edge-
sensitive interrupts. Like the timers, these usually are synchronized to the in-
ternal processor clock, which limits the minimum pulse-width that will be reliably
recognized as an interrupt request.

Design Shortcuts

It is sometimes possible to simplify a design by taking some shortcuts with the hard-
ware. We’ll discuss a few such shortcuts in this section.

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