Clock speed

This document describes the format for clock speed load values. The clock speed can be set by writing to the first two registers at Memory-mapped_I/O_ports#900B0000_-_Power_management. Clock speeds are in units of MHz. Most of this information is derived from Nover and nspire_emu source code.

CX model

Bit	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	..	30	31
		Base/CPU value							48MHz base				CPU/AHB value			Base value							Unknown

Base to CPU ratio (if 48MHz base)	1 << (Unknown)
CPU to AHB ratio (if 48MHz base)	2
Base clock speed (if 48MHz base)	48
Base to CPU ratio	(Base/CPU value)*2 or 2 if 0
CPU to AHB ratio	(CPU/AHB value) + 1
Base clock speed	6 * (Base value)
CPU clock speed	(Base clock speed) / (Base to CPU ratio)
AHB clock speed	(CPU clock speed) / (CPU to AHB ratio)
APB clock speed	(AHB clock speed) / 2

The unknown quantity only ever seems to be either 1 or 2. It’s probably a multiplier.

Classic model

Bit	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
		Base/CPU value							27MHz base				CPU/AHB value				Base value

Base to CPU ratio	(Base/CPU value) * 2
CPU to AHB ratio	(CPU/AHB value) + 1
Base clock speed (if 27MHz base)	27
Base clock speed	300 - 6 * (Base value)
CPU clock speed	(Base clock speed) / (Base to CPU ratio)
AHB clock speed	(CPU clock speed) / (CPU to AHB ratio)
APB clock speed	(AHB clock speed) / 2