Many chips specify that they can be accessed only at specified intervals. For example, the Zilog Z8530 SCC has a “write recovery time” of 1.6 microseconds. This specification means that a delay must be enforced with drv_usecwait(9F) when writing characters with an 8530. In some instances, the specifications do not make explicit what delays are needed, so the delays must be determined empirically.
Be careful not to compound delays for parts of devices that might exist in large numbers, for example, thousands of SCSI disk drives.