Acorn Computers Limited BBC A, Руководство по оборудованию, Страница: 38 / 47

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Normally devices attached to the 1MHz bus should decode the address lines A~ to A7 and also the
NPGFC line (FRED). In most cases the signal on the NPGFC line will need to be "cleaned up" by one of
the circuits shown in figure 3.5 below.
The reason that the NPGFC and NPGFD signals need to be cleaned up is because of the difference in
the speed that the 1MHz bus is running at, compared to the speed at which the machines internal 65~2A
processor is running. The machines internal processor runs at a clock speed of 2 MHz unless a slow
speed device, such as the 1 MHz bus, is being accessed. Circuitry within the BBC Microcomputer is used
to effectively "stretch" the 2 MHz normal 6502A clock cycle into the 1 MHz clock (1 MHz E) cycle,
required by 1 MHz peripheral devices attached to the 1MHz bus. This circuitry is described in some detail
above.
The NPGFC signal (FRED) produced by the microcomputer's internal circuitry suffers from two distinct
problems, these are "glitches" and "double accessing
Refer to diagram 3.4 below.
3.20 Glitches.
The machines internal address bus will normally only change state when the 2MHz clock (2MHz E) is
low. It can be seen from figure 3.4, however, that when the address bus changes state the 1MHz E clock
is alternately high then low. This results in low-going "glitches" being produced in the NPGFC signal.
These