mmc接口（1）——emmc引脚驱动能力

EMMC IO Operation Mode

The EMMC IO PAD is a multipurpose PAD which can be programmed to operate in different
modes:
1、Output with predetermined source/ sink impedance
2、Open drain
3、Input
4、Tristate
5、Weak pull up or pull down
The PAD mode of operation is determined by ‘phyctrl_dren_xxx’, ‘phyctrl_oden_xxx’, ’phyctrl_ren_xxx’ and ‘phyctrl_pu_xxx’ control signal(xxx equals to dat or cmd).
The ‘phyctrl_dr_ty[2:0]’ are used to program the drive strength of the IO in push/pull mode of operation by programming the source/sink impedance of EMMC51 IO. The PAD source/sink impedance can be programmed to 50, 33, 66, 100 or 40 Ohms. The source/sink impedance variation across PVT exceeds 25 % of its nominal value. Trimming bits are provided to greatly reduce the variation. phyctrl_rtrim[3:0] control are used to trim the selected source/sink impedance value. The dynamic range of the trim is +/- 25%. The power on default setting of phyctrl_rtrim[3:0] equals 4’b1110.

所以我们需要关注：
1、phyctrl_dren_xxx, phyctrl_oden_xxx, phyctrl_ren_xxx and phyctrl_pu_xxx control signal
2、phyctrl_dr_ty[2:0]
3、phyctrl_rtrim[3:0]

如图所示，不同的IO mode对应了一组寄存器的状态

IO的阻抗则由phyctrl_dr_ty[2:0]控制