Hardware Manual - RTD Embedded Technologies, Inc.

8254 Timer/Counters
Two 8254 programmable interval timers provide six (three each) 16-bit, 8 MHz timer/counters to support a wide
range of board operations and user timing and counting functions.
One 8254 is the Clock TC, which is used for board operations. Two of its 16-bit timer/counters, Counter 0 and
Counter 1, are cascaded and used internally for the pacer clock. The pacer clock is described in Chapter 5. The
third timer/counter in the Clock TC, Counter 2, is the burst clock. Figure 3 shows the Clock TC circuitry.
The other 8254 is the User TC, which has two 16-bit timer/counters (Counter 0 and Counter 1) for user functions
and one 16-bit timer/counter (Counter 2) for the sample counter. Figure 4 shows the User TC circuitry.
Each 16-bit timer/counter has two inputs, CLK in and GATE in, and one output, timer/counter OUT. Each can
be programmed as binary or BCD down counters by writing the appropriate data to the command word, as
described in Chapter 4. The command word also lets you set up the mode of operation. The six programmable
modes are:
Mode 0
Mode 1
Mode 2
Mode 3
Mode 4
Mode 5
Event Counter (Interrupt on Terminal Count)
Hardware-Retriggerable One-Shot
Rate Generator
Square Wave Mode
Software-Triggered Strobe
Hardware Triggered Strobe (Retriggerable)
Note The pacer clock, burst clock, and sample counter should be programmed for Mode 2 operation.
COUNTER
0
CLK
GATE
OUT
8 MHz (OSC)
PACER CLOCK
GATE CONTROL
COUNTER
1
CLK
GATE
OUT
16-BIT PACER CLOCK
32-BIT PACER CLOCK
COUNTER
2
CLK
GATE
OUT
8 MHz (OSC)
BURST GATE CONTROL
BURST CLOCK
Figure 3
Clock TC Circuit Block Diagram
BDM-610010034 Rev C Chapter 1: Introduction 9