1. The Silicon Laboratories C8051F020
Enhanced 8051 Part 2
Timers I
ECE/CS-352: Embedded Microcontroller Systems

2. ECE/CS-352: Embedded Microcontroller Systems
SFRs for Subsystems
Control registers:
XXXCN (WDTCN, ADC0CN, DAC0CN, TMR3CN)
Configuration registers:
XXXCF (ADC0CF, AMX0CF, EMI0CF)
Data registers for subsystems:
ADC0H, ADC0L, DAC0H, DAC0L
Many others.....
ECE/CS-352: Embedded Microcontroller Systems

3. ECE/CS-352: Embedded Microcontroller Systems
System Clocks
Internal clock oscillator
default at reset
2 MHz default frequency
configured by SFR OSCICN
External oscillator installed on board
MHz
configured by SFR OSCXCN
ECE/CS-352: Embedded Microcontroller Systems

4. Internal Osc Configuration OSCICN
Missing clock
detector enable
Internal Osc. frequency ready
Internal Osc. freq. control bits
00 – 2MHz
01 – 4 MHz
10 – 8 MHz
11 – 16 MHz
system clock select
0 = internal
1 = external
(internal by
default)
Default Sysclk
f = 2 MHz
T = ??
Internal Osc. enable
(enabled by default)
ECE/CS-352: Embedded Microcontroller Systems

5. External Osc. Configuration OSCXCN
External Osc. freq. control bits
Depends on frequency range of operation
External Osc. mode bits
00x: Off. XTAL1 pin is grounded internally.
010: System Clock from External CMOS Clock on XTAL1 pin.
011: System Clock from External CMOS Clock on XTAL1 pin divided by 2.
10x: RC/C Oscillator Mode with divide by 2 stage.
110: Crystal Oscillator Mode
111: Crystal Oscillator Mode with divide by 2 stage.
Crystal Osc.
valid flag
ECE/CS-352: Embedded Microcontroller Systems

6. Development Board Oscillator
ECE/CS-352: Embedded Microcontroller Systems

7. To use 22.1184 MHz external oscillator
110: Crystal Oscillator Mode
111: f > 6.7 MHz
Step 1. Configure the external oscillator.
Step 2. Wait at least 1 ms.
Step 3. Poll for XTLVLD => ‘1’.
Step 4. Switch the system clock to the external oscillator.
ECE/CS-352: Embedded Microcontroller Systems

8. ECE/CS-352: Embedded Microcontroller Systems
Instruction Timing
C8051F020 – Pipelined architecture
Machine cycle = clock cycle
Most instructions take 1 or 2 clock cycles
Total of 109 distinct instruction types
Summary of instruction timing:
ECE/CS-352: Embedded Microcontroller Systems

9. Delay Loops clr A djnz A, $ ; 2 cycles if no jump, ; 3 cycles if jump
ECE/CS-352: Embedded Microcontroller Systems

10. ECE/CS-352: Embedded Microcontroller Systems
Sample Configuration
Use MHz external oscillator
Main:
mov WDTCN, #0deh ; disable watchdog timer
mov WDTCN, #0adh
mov OSCXCN, #67h ; enable external crystal
; oscillator at MHz
clr A ; wait at least 1ms
djnz A, $ ; wait ~510us (255 x 4 x 0.5us) = 510 us)
djnz A, $ ; wait ~510us
osc_wait: ; poll for XTLVLD-->1
mov a, OSCXCN
jnb acc.7, osc_wait
orl OSCICN, #08h ; select external oscillator as
; system clock source
ECE/CS-352: Embedded Microcontroller Systems

11. ECE/CS-352: Embedded Microcontroller Systems
Timers
Original 8051 has 2 Timers, T0 and T1
Can think of timers as binary counters
Clock is derived from system clock (frequency is configurable)
Timer registers can be read via mov instructions
Events (interrupts or flag bits set) occur when timers overflow
Can be used to create waveforms, measure time intervals, set frequency of events, etc.
ECE/CS-352: Embedded Microcontroller Systems

12. ECE/CS-352: Embedded Microcontroller Systems
C8051F020 Timers
Size of timers can be configured.
Timers can count from arbitrary initial value with the “Auto-reload” feature.
“Capture” allows the timer to be “frozen” when an event occurs.
Timers 2 and 4 can be used to set baud rate for uarts.
ECE/CS-352: Embedded Microcontroller Systems

13. ECE/CS-352: Embedded Microcontroller Systems
Timer Clock Sources
CKCON: Clock Control Register
Timer0
Timer clock selects
0 = system clock  12 (for compatibility with original 8051,
and for slow stuff)
1 = system clock
Timer4
ECE/CS-352: Embedded Microcontroller Systems

14. ECE/CS-352: Embedded Microcontroller Systems
Configuring Timers
TCON and TMOD used to configure Timer0 and Timer1
TCON: Timer Control Register
These deal with external interrupts
Timer 1 Overflow Flag
Timer 1 run control
0 = disable Timer 1
1 = enable Timer 1
ECE/CS-352: Embedded Microcontroller Systems

15. ECE/CS-352: Embedded Microcontroller Systems
Configuring Timers
TMOD: Timer Mode Register
Timer 1
Timer 0
GATE1: 0 = timer enabled with TR1 only
1 = timer enabled only when TR1=1 and \INT=1
C/T1: = timer incremented by clock input
1 = counter incremented by edge on external T1 pin
ECE/CS-352: Embedded Microcontroller Systems

16. Using Timers For Periodic Events Using Polling
Set mode to be timer – use appropriate clock and timer size
In main program
Check for overflow flag (polling)
When overflow flag occurs
Do event
Reset timer overflow flag
Reset timer initial value (unless using auto-reload or initial value of 0 is OK)
Continue checking
ECE/CS-352: Embedded Microcontroller Systems

17. Using Timers For Periodic Events Using Interrupts
Set mode to be timer, use appropriate clock and timer size
Set timer to interrupt on overflow
In Interrupt Service Routine
Do event
Reset interrupt flag
Reset timer to initial value (unless using auto-reload)
Return from interrupt
ECE/CS-352: Embedded Microcontroller Systems

18. Using Timers as Counter
Set mode to be counter, with appropriate size (8, 13 or 16 bits)
Initialize counter to zero
Enable counter
Counter can be read and reset by main program.
ECE/CS-352: Embedded Microcontroller Systems

19. Using Timers to Measure Time
Configure timers with mode 0 or 1 (13 or 16-bit timer) and desired system clock.
In main program, wait for initial event.
Start the timer. 
Each time the timer overflows, a register should be incremented using an interrupt service routine.
When the second event occurs, disable the interrupts and the timer in the main program.
ECE/CS-352: Embedded Microcontroller Systems

20. ECE/CS-352: Embedded Microcontroller Systems
Configuration Tool
Fortunately, there is a tool to help configure all the subsystems in the 8051.
This makes things a lot easier.
To access the tool, go to Silicon Labs – Configuration Tool 2
ECE/CS-352: Embedded Microcontroller Systems