IBM 76, 5.5 Clock Generator Operations

76
CHAPTER 5 CLOCK GENERATOR
5.5 Clock Generator Operations
The clock generator generates the following various types of clocks and controls the CPU operating mode
including the standby mode.
• Main system clock fXX
• CPU clock fCPU
• Clock to peripheral hardware
The following clock generator functions and operations are determined with the processor clock control register
(PCC) and the oscillation mode selection register (OSMS).
(a) Upon generation of RESET signal, the lowest speed mode of the main system clock (12.8
µ s when operated
at 5.0 MHz) is selected (PCC = 04H, OSMS = 00H). Main system clock oscillation stops while low level is
applied to RESET pin.
(b) The six types of CPU clocks (0.4
µ s. 0.8 µ s, 1.6 µ s, 3.2 µ s, 6.4 µ s, 12.8 µ s : 5.0 MHz) can be selected by setting
the PCC and OSMS.
(c) Two standby modes, the STOP and HALT modes, are available.
(d) The main system clock is divided and supplied to the peripheral hardware. Thus, the peripheral hardware
also stops if the main system clock is stopped. (Except external input clock operation)