The present invention generally relates to memory controllers operating in a system containing a variable system clock. The memory controller may exchange data with a processor operating at a variable processor clock frequency. However the memory controller may perform memory accesses at a constant memory clock frequency. Asynchronous buffers may be provided to transfer data across the variable and constant clock domains. To prevent read buffer overflow while switching to a lower processor clock frequency, the memory controller may quiesce the memory sequencers and pace read data from the sequencers at a slower rate. To prevent write data under runs, the memory controller's data flow logic may perform handshaking to ensure that write data is completely received in the buffer before performing a write access.