STR711 PLL won't lock

Development Boards MicroMod
1

I’m porting our ARM development system from the STR-710EVAL board to the Olimex STR-P711 board. I’m trying to program the PLL for 48 MHz MCLK and 24 MHz PCLK1,2. This worked just fine on the STR-710EVAL (which has a 16 MHz CK input) with the following register settings:

MDIVR 0000

PDIVR 0101

PLL1CR 0051

CFR 8009

Now I’m using these values for the STR-P711 (4 MHz CK input):

MDIVR 0000

PDIVR 0101

PLL1CR 0050

CFR 0009

The LOCK bit in CFR never goes true. What have I overlooked?