Pacer Trigger Source

The counter 1 and counter 2 are cascaded together to generate the timer pacer trigger of A/D conversion. The frequency of the pacer trigger is software controllable. The maximum pacer signal rate is 2MHz/4=500K which excess the maximum A/D conversion rate of the ACL-8112. The minimum signal rate is 2MHz/65535/65535, which is a very slow frequency that user may never use it.

General Purpose Timer / Counter

The counter 0 is free for users' applications. The clock source, gate control signal and the output signal is send to the connector CN3. The general purpose timer / counter can be used as event counter, or used for measuring frequency, or others functions. See the 'Timer/Counter Applications' section for examples.

I/O Address

The 8254 in the DAS BOARD occupies 4 I/O address as shown below.

BASE + 0	LSB OR MSB OF COUNTER 0
BASE + 1	LSB OR MSB OF COUNTER 1
BASE + 2	LSB OR MSB OF COUNTER 2
BASE + 3	CONTROL BYTE

The programming of 8254 is control by the registers BASE+0 to BASE+3. The functionality of each register is specified this section For more detailed information, please refer handbook of 8254 chip.

Control Byte

Before loading or reading any of these individual counters, the control byte (BASE+3) must be loaded first. The format of the control byte is:

Bit	7	6	5	4	3	2	1	0
	SC1	SCO	RL1	RLO	M2	M1	MO	BCD

SC1 & SCO - Select Counter ( Bit7 & Bit 6)

SC1	SCO	COUNTER
0	0	Select Counter 0
0	1	Select Counter 1
1	0	Select Counter 2
1	1	ILLEGAL

RL1 & RLO - Select Read/Load operation ( Bit 5 & Bit 4)

RL1	RLO	OPERATION
0	0	COUNTER LATCH FOR STABLE READ
0	1	READ/LOAD LSB ONLY
1	0	READ/LOAD MSB ONLY
1	1	READ/LOAD LSB FIRST, THEN MSB

M2, M1 & MO - Select Operatinq Mode ( Bit 3, Bit 2, & Bit 1)

M2	M1	MO	MODE
0	0	0	0
0	0	1	1
X	1	0	2
X	1	1	3
1	0	0	4
1	0	1	5

BCD - Select Binary/BCD Countina ( Bit 0)

0	16-BITS BINARY COUNTER
1	BINARY CODED DECIMAL (BCD) COUNTER (4 DIGITAL)

Note The count of the binary counter is from 0 up to 65,535 and the count of the BCD counter is from 0 up to 9,999.

In 8254, six operating modes can be selected, they are:

• Mode Definitions

• Mode 0 Interrupt on Terminal Count

• Mode 1 Programmable One-Shot.

• Mode 2 Rate Generator.

• Mode 3 Square Wave Rate Generator.

• Mode 4 Software Triggered Strobe.

• Mode 5 Hardware Triggered Strobe.

All detailed description of these six modes are written in Intel Microsystems Components Handbook Volume II Peripherals.