The System Control register controls the system mode and also contains several miscellaneous control bits. The System Control register is located at index -1Eh and is defined as follows:
Bit Reset Value R/W Mode Description
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[0] 0 W SHT - CPU shutdown; Setting this bit stops clocks to
the CPU. The system enters light sleep mode if the
DON bit is set and will enter deep sleep mode if the
DON bit is clear. This bit is set only, it is
automatically cleared by a wakeup.
[1] 0 R/W BUP - Backup; This status bit is set when the system
has been in backup mode. It should be cleared by software
[2] 0 R/W DON - Display ON; Setting this bit turns on the display.
[3] - - unused
[4:5] 00 R/W SPD[0:1] - Speed bits; Should be set for crystal used
00 - 10.738636 MHz
01 - 15.836774 MHz
10 - 21.477272 MHz
11 - 31.673550 MHz
[6:7] 00 R/W VOL[0:1] - Speaker volume bits
00 - off (0V)
01 - soft (3V)
10 - medium (5V)
11 - loud (8V)