Programmable Timer Operation There are three internal l6-bit programmable timers in the 8~186 microprocessor, although only two are pinned out. The other timer is for real time applications and generating delayed signals. One of the two pinned out timers is used for generating a baud clock signal at 1.2288 Mhz. The other pinned out timer is used for the system clock. The timers are controlled by eleven l6-bit registers in the internal peripheral control block, see Table 5-3. The count register contains the current value of the timer. It can be read or written at any time independent of whether the timer is run- ning or not. The value of the register will be incremented for each timer event.
Each of the timers is equipped with a MAX COUNT register, which defines the maximum count the timer will reach. After reaching the MAX COUNT register value, the timer count value will reset to zero during that same clock, i.e., the maximum count value is never stored in the count register itself. The timers have several programmable options: o All three timers can be set tc halt or continue on a terminal count. o Timers ~ and 1 can select between internal and external clocks, alternate between MAX COUNT registers and be set to retrigger on external events. o The timers may be programmed to cause an interrupt on terminal count.
Table 5-3 Timer Control Register Offsets ADDRESS (HEX) FUNCTION TIMER NO. FF5~H FF52H FF54H FF56H FF58H FF5AH FF5CH FF5EH COUNT REGISTER MAX COUNT A MAX COUNT B MOD E/ CONTROL COUNT REGISTER MAX COUNT A MAX COUNT B MODE/CONTROL COUNT REG ISTER MAX COUNT A MODE/CONTROL TIMER ~ TIMER ~ TIMER ~ TIMER ~ TIMER 1 TIMER 1 TIMER 1 TIMER ~ TIMER 2 TIMER 2 TIMER 2 FF6~H FF62H FF66H 5-6