PIC16F877-20/P Microchip Technology, PIC16F877-20/P Datasheet - Page 3

• Manufacturer Part Number: PIC16F877-20/P
• Description: IC MCU FLASH 8KX14 EE 40DIP
• Manufacturer: Microchip Technology
• Series: PIC® 16Fr

Specifications of PIC16F877-20/P

• Core Size: 8-Bit
• Program Memory Size: 14KB (8K x 14)
• Core Processor: PIC
• Speed: 20MHz
• Connectivity: I²C, SPI, UART/USART
• Peripherals: Brown-out Detect/Reset, POR, PWM, WDT
• Number Of I /o: 33
• Program Memory Type: FLASH
• Eeprom Size: 256 x 8
• Ram Size: 368 x 8
• Voltage - Supply (vcc/vdd): 4 V ~ 5.5 V
• Data Converters: A/D 8x10b
• Oscillator Type: External
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 40-DIP (0.600", 15.24mm)
• Controller Family/series: PIC16F
• No. Of I/o's: 33
• Eeprom Memory Size: 256Byte
• Ram Memory Size: 368Byte
• Cpu Speed: 20MHz
• No. Of Timers: 3
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: 444-1001 - DEMO BOARD FOR PICMICRO MCU
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

1. Asynchronous Counter
EXAMPLE 1:
 2010 Microchip Technology Inc.
Update:
Critical Timing of code sequence for instructions following last write to TMR1L or TMR1H.
When Timer1 is started or updated, the timer
needs to see a falling edge from the external clock
source before a rising edge can increment the
counter. If writes to TMR1H and TMR1L are not
completed while the external clock pulse is still
high, Timer1 will miss counting the first clock pulse
after the update.
When using an external crystal, the pulse width
from rising to falling edge is temperature depen-
dent and may decrease with temperature. As a
result, the timer may require an additional oscilla-
tion to overflow.
Code examples are given for the affected devices:
• PIC12/14/16/17 devices – Example 1 and
• PIC18 devices – Example 3
Both examples include code to wait for Timer1 to
increment twice between the RTCisr and Update
labels.
BTFSC
GOTO
BTFSS
GOTO
BCF
BSF
BCF
BSF
BSF
Example 2
TMR1L,0
$-1
TMR1L,0
$-1
T1CON,TMR1CS
TMR1H,7
T1CON,TMR1ON
T1CON,TMR1C
T1CON,TMR1ON
PIC12/14/16/17 CODE EXAMPLE FOR 1 SECOND OVERFLOW PERIOD WITH 32.786
kHZ OSCILLATOR
;Timer has just incremented, 31 μs before next rising edge to
;complete reload
;Select system clock for Timer1
;Timer1 high byte 0x80
;Timer1 off
;Select external crystal
;Timer1 on
In PIC18 devices, it is not possible to reliably
update Timer1 in a low-priority interrupt. A high-
priority interrupt could occur at any time and
unexpectedly delay the TMR1 update.
PIC18 devices also include Timer3 which is
functionally identical to Timer1.
Work around
Switching Timer1 to the main system oscillator
after reloading, the timer ensures the timer will see
a falling edge before switching back to the external
clock source.
Due to the time from Timer1 overflow to the reload
being application specific, wait for the timer to
increment before beginning the reload sequence.
This ensures the timer does not miss a rising edge
during reload. The timing of the clock source
changing is critical and is detailed in Example 1
and Example 2.
TIMER1 MODULE
DS80329B-page 3