Field Name Bits Default Description
CntDownSelect 0 0b 0 – Binary countdown
1 – BCD countdown
ModeSelect 3:1 000b 000 – Asserts OUT signal at end of count
001 – Hardware re-triggerable one-shot
010 – Rate generator
011 – Square wave output
100 – Software triggered strobe
101 – Hardware triggered strobe
110 – 111 – not used
CmmandSelect 5:4 00b 00 – Counter latch command
01 – Read/write least significant byte
10 – Read/write most significant byte
11 – Read/write least, and then most significant byte
CounterSelect 7:6 00b 00 - Select counter 0
01 - Select counter 1
10 - Select counter 2
11 – Read back command
Tmr1CntrlWord register: This is the control word to access the 8254 timer 1. It is used to select which counter will
be accessed and how it will be accessed. This register specifies the counter, the operating mode, the order and
size of the count value, and whether it counts down in a 16 bit or BCD format.
If a counter is programmed to read or write two-byte counts, the following precaution applies: A program must not
transfer control between writing the first and second byte to another routine which also writes into that same
counter; otherwise, the counter will be loaded with an incorrect value. The count must always be completely loaded
with both bytes.
Nmi_Status - RW – 8 bits - [IO_Reg: 61h] Field Name Bits Default Description
SpkrEnable 0 0b 0 - Disable counter 2
1 - Enable counter 2
SpkrTmrEnable 1 0b 0 - Speaker timer off
1 - Speaker timer on
Parity_Nmi_En 2 1b 0 - Enable Parity Error to NMI generation (from SERR# or
PERR#)
1 - Disable Parity Error to NMI generation and clear bit 7
Iochk_Nmi_En 3 1b 0 - Enable IoChk to NMI generation
1 - Disable IoChk to NMI generation
RefClk 4 - The output of the counter 1 (8254). (Read-only)
SpkrClk 5 - The output of the counter 2. [Read-only]
IoChk_Nmi 6 - NMI is triggered by serial IOCHK. [Read-only]
ParErr_Nmi 7 - NMI is caused by parity error (either PERR# or SERR#).
[Read-only]
Nmi_Status register: Independent read and write registers will be accessed at this port. When writing to port 61H,
bits[3:0] allow software to enable/disable parity error NMI's and control the speaker timer. When reading port 61H,
status on parity errors, speaker count, speaker control and refresh cycles is returned.
Nmi_Enable - RW – 8 bits - [IO_Reg: 70h] Field Name Bits Default Description
RTC Address Port 6:0 00h This is used with either internal RTC or external RTC
NmiEnable 7 0b 0 - NMI enable
1 - NMI disable [Write-only]
Nmi_Enable register
RtcDataPort - RW – 8 bits - [IO_Reg: 71h]
Field Name Bits Default Description
RTC Data Port 7:0 00h This is used with either internal RTC or external RTC