AN2295 Freescale Semiconductor / Motorola, AN2295 Datasheet - Page 28

• Manufacturer Part Number: AN2295
• Description: Developers Serial Bootloader for M68HC08 and HCS08 MCUs
• Manufacturer: Freescale Semiconductor / Motorola

MCU Slave Software
Internal Clock Generator (ICG) — Initialization
The ICG is simple to initialize. Because the ICG is active and the clock monitor is disabled after reset, the
only action required is the modification of the ICG multiply register. Then, the ICGS flag (bit 2) of the ICG
control register indicates whether ICG is stable after the frequency change.
ICGMRINIT
LOOP:
Internal Clock Generator — Trimming
Even though the trimming routine is in ROM, a small bug renders this code unusable. So the source code
has been taken and inserted in the bootloader code.
Although AN1831/D provides the procedure for calculating the trim factor from the measured CPU speed,
the code itself omits the final doubling of the number of cycles.
* FOLLOWING LOOP IS EXECUTED UNTIL THE END OF THE BREAK SIGNAL. THE BREAK
* SIGNAL LASTS 10 BIT TIMES. IF COMMUNICATING AT f OP /256 BPS, THEN 10 BIT
* TIMES IS 2560 CYCLES. EACH TIME THROUGH THE LOOP IS 10 CYCLES, SO WE
* EXPECT TO EXECUTE THE LOOP 256 TIMES IF THE KX8 IS IN SYNC SERIALLY WITH
* THE HOST. IF WE STAY IN THE LOOP FOR > 256 LOOP CYCLES, THEN THE KX8
* MUST BE RUNNING FASTER THAN EXPECTED, AND NEEDS TO BE SLOWED DOWN. IF WE
* STAY IN THE LOOP FOR < 256 LOOP CYCLES THEN THE KX8 MUST BE RUNNING SLOWER
* THAN EXPECTED AND NEEDS TO BE SPEEDED UP. THE AMOUNT THAT WE CHANGE THE
* CPU SPEED IS EQUAL TO THE NUMBER OF LOOP CYCLES OVER OR UNDER 256. SO IF
* WE GO THROUGH THE LOOP 240 TIMES, THEN WE ARE RUNNING
* (256-240)/256 = 6.25% FAST. EACH INCREMENTAL CHANGE WE MAKE TO THE TRIM REGISTER
* (ICGTR) WILL MAKE A 0.195% CHANGE TO THE INTERNAL CLOCK. THAT IS, INCREMENTING
* THE REGISTER BY ONE OVER THE DEFAULT VALUE OF $80 STORED THERE WILL
* DECREASE THE INTERNAL CLOCK BY 0.195%, AND VICE VERSA.
* NOW EACH EXECUTION OF THE LOOP OVER OR UNDER WHAT IS EXPECTED (256 TIMES)
* REPRESENTS AN ERROR OF 1/256 = .391% ERROR. SO WE'LL NEED TO DOUBLE THE
* NUMBER OF LOOP CYCLES AND USE THIS NUMBER TO CORRECT THE TRIM REGISTER.
* OUR PRECISION FOR TRIMMING IS THEREFORE 0.391%.
The actual code adds an ASLA instruction which doubles the trim factor before the actual write to the ICG
trim register.
ICGTRIM:
MONPTB4:
CHKPTB4:
28
MOV
BRCLR
CLRX
CLRH
BRSET
BRSET
AIX
BRA
EQU
#ICGMRINIT,ICGMR
2,ICGCR,LOOP
4,PTB,MONPTB4
4,PTB,BRKDONE
#1
CHKPTB4
Developer’s Serial Bootloader for M68HC08 and HCS08 MCUs, Rev. 6
$20
;WAIT FOR BREAK SIGNAL TO START
;(5) GET OUT OF LOOP IF BREAK IS OVER
;(2) INCREMENT THE COUNTER
;(3) GO BACK AND CHECK SIGNAL AGAIN
; set 9.8304MHz BUS clock
; wait until ICG stable
Freescale Semiconductor