Here's an extract from the 18F452 datasheet;
Timer2 is an 8-bit timer with a prescaler and a postscaler. It can be used as the PWM time base for the PWM mode of the CCP module(s). The TMR2 register is readable and writable and is cleared on any device Reset. The input clock (FOSC/4) has a prescale option of 1:1, 1:4 or 1:16, selected by control bits T2CKPS1:T2CKPS0 (T2CON).
The Timer2 module has an 8-bit period register, PR2. Timer2 increments from 00h until it matches PR2 and then resets to 00h on the next increment cycle. PR2 is a readable and writable register. The PR2 register is initialized to FFh upon Reset.
The match output of TMR2 goes through a 4-bit postscaler (which gives a 1:1 to 1:16 scaling inclusive) to generate a TMR2 interrupt (latched in flag bit, TMR2IF (PIR1)). Timer2 can be shut-off by clearing control bit, TMR2ON (T2CON), to minimize power consumption.
Basically the TMR2 will continue to cycle until it equals the register PR. How fast it counts depends on the pre and post scaler settings. When TMR2 matches PR2 an overflow flag is triggered, along with the interrupt.
TMR2 is great for small delays and PWM generation. Here's an example of how to set it up, the settings for PR2 and prescalers were found using the TMR2 calculator.
Note the PIC's power supply/oscillator are not shown
Device = 18F452 Clock = 20 Dim TMR2IE As PIE1.1, // TMR2 interrupt enable TMR2IF As PIR1.1, // TMR2 overflow flag TMR2ON As T2CON.2, // Enables TMR2 to begin incrementing mS As Word, // mS register Last_mS As Word, // mS polling register Signal_Pin As PORTB.0 // Signal output to frequency meter Interrupt TMR2_Interrupt() Save(0) // Back up system variables If TMR2IF = 1 Then // Check if the interrupt was from TMR2 TMR2IF = 0 // Clear the TMR2 interrupt flag Inc(mS) // Increment the mS counter EndIf // Restore // Restore system variables End Interrupt Private Sub TMR2_Initialize() TMR2ON = 0 // Disable TMR2 TMR2IE = 0 // Turn off TMR2 interrupts PR2 = 249 // TMR2 Period register PR2 T2Con = %00100001 // T2CON 0:1 = Prescale // 00 = Prescaler is 1:1 // 01 = Prescaler is 1:4 // 1x = Prescaler is 1:16 // 3:6 = Postscale // 0000 = 1:1 postscale // 0001 = 1:2 postscale // 0010 = 1:3 postscale... // 1111 = 1:16 postscale TMR2 = 0 // Reset TMR2 Value TMR2IE = 1 // Enable TMR2 interrupts TMR2ON = 1 // Enable TMR2 to increment Enable(TMR2_Interrupt) End Sub // Start Of Program... mS = 0 // Reset mS register Last_mS = 0 // Reset mS polling register TMR2_Initialize // Setup and enable TMR2 Low(Signal_Pin) // Make the signal an output, and set it low While 1 = 1 // Create an infinite loop Repeat // Poll the mS register and wait for a change Until mS <> Last_mS // Last_mS = mS // Update the polling register Signal_Pin = 1 // Create a pulse for the frequency counter Signal_Pin = 0 Wend
pic18f46j50 adc compiler error - jon chandler Monday, 21 May 2012 22:11 [16 replies]
Nuffink works... - majenko Monday, 21 May 2012 21:40 [5 replies]T-Pad, H-Pad, LP & HP Filter BNC Board - jon chandler Monday, 21 May 2012 14:11 [17 replies]
My eCandle - mrdeb Sunday, 20 May 2012 17:58 [7 replies]Free electricity - majenko Sunday, 20 May 2012 16:57 [1 reply]
Create / Edit Article 
Resource Links 
DOCman 
My Details 
Member List 
Newsletters 
Contact Admin 
